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AAWSAP DIRD, Inertial Electrostatic Confinement Fusion, March 2010

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DOW-UAP-D125

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Document, 72 pages

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Department of War

Date

3/10/10

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Las Vegas, Nevada

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Release 6, September 18, 2026

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The government's description

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed.

This DIRD surveys inertial electrostatic confinement (IEC) fusion, a relatively unconventional fusion concept that uses electric fields rather than the more established magnetic or laser-based approaches to confine ions, and it reviews both the underlying physics and the experimental work associated with the concept. The report emphasizes that IEC may have nearer-term value as a compact neutron, proton, or x-ray source and as a platform for studying experimental fusion approaches, while also presenting more ambitious possibilities such as aneutronic power generation and propulsion applications. At the same time, it makes clear that the concept remained far from practical fusion power, with experimental devices operating several orders of magnitude below breakeven and with major unresolved issues involving confinement, losses, grid damage, and scale-up. Overall, the document treats IEC as a technically interesting but still highly speculative path toward fusion energy, while suggesting that its more limited spin-off applications were more plausible in the near term than its long-range power generation or propulsion applications.

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Defense
Intelligence
Reference
Document
Acquisition Threat Support
10 March 2010
ICOD : 1 Decem ber 2009
DIA-08- 1003-006

Inertial Electrostatic
Confinement Fusion

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Inertial Electrostatic Confinement Fusion

Prepared by:
Acquisition Support Division (DW0-3)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Author:

AAP Person 70
Administrative Note
COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized.

This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace
Weapon System Applications (AAWSA) Program. Comments or questions pertaining to
AAWSA Prog ram
this document should be addressed to !AAP Person 1
Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000 , Washington,
DC 20340- 5100 .

I

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Contents
Preface ................................................................................................................. vii
Section I. IEC Background and Basics .................................................................... 1
IEC Background ....................................................................................... ,.,1••········· 3
IEC Basics ............. .............................................................. ................................ 5
Bussard HEPS (or Polywell) Concept ............................................................ 10
Barnes Nebel Penning Trap .......................................................................... 10
Nebel POPS Device ..............................................................................,......... 10
Miley's "Ion Injected" Device ....................................................................... 11
Closing Remarks ......... .......... ................................................................... 11 ,,, •••••• 12
References ...................................................................................................... 13
Section II. Select Experiments ............................................................................. 14
Closing Comments ............................................................................................ 20
References ...................................................................................................... 20
Section III. Other Geometries .............................................................................. 21
Cylindrical IECs ............................................................................................... 21
Electrically-Driven IEC Jet Thruster................................................................... 22
Relation to Other Prior Thrusters ................................................................. 23
JET Extraction From a Spherical IEC ............................................................. 24
Description of the IEC Jet Thruster ............................................................. 24
Comparison to a Conventional "Plasma" Thruster ........................................ 24
Jet Extraction .............................................................................................. . 26
Experimental Jet Design and Performance ................................................... 26
Scale-up to p- 11 B IEC Space Power Unit/Thruster ........................................ 28
The Dipole Assisted IEC (DaIEC) .................................................................. 29
DAIEC Experiments .......................................................................................... 30
Khachan's Studies at U of Sydney .................................................................... 31
Concluding Remarks......................................................................................... 32
References ........................ . .....................................................................

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Section IV. IEC Theory ......................................................................................... 34
Potential Well Structure ................................................................................... 38
Tzonev et al. - Deep Well Study ....................................................................... 39
Momota et al. - Study of Virtual Electrode Strudure ........................................ 41
Kim - Stability Analysis .................................................................................... 41
Rider - Energy Balance Study .......................................................................... 43
Neutron Source Simulations ............................................................................... 44

References ....................................................................................................... 46
Section V. Potential Applications .......................................................................... 46
Neutron/proton/Xray Sources ............................................................... ......... 46
Integrated X-Ray /Neutron Sources ................................................................ 47
Nuclear and Chemical Explosive Detection Techniques ................................ 47
Overview for Weapons/Nuclear Material Simulation Articles ............. .......... 48
Pulsed Power for the Inspection Station ............................................. ......... 49
Design of the Total Integrated Interrogation System ................................... 50
Detector Array and Analysis System ............................................................ 52
Space Propulsion .............................................................................................. 54
Magnetically-Channeled Spherical IEC Array (MCSA) Concept ......................... 56
Recirculation of Radial Belt Cone Losses ..................................................... 57
Retrapping of Axial-Loss Particles................................................................ 58
Concluding Remarks ........................................................................................... 59

References ................................................................................................ ........ 59
Section VI. Possible Next Step Breakeven Experiment ......................................... 60
Demonstration of Net Energy Gain using IEC Aneutronic Fusion ...................... 60
Vision of a Future p- 11 8 Fusion Plant .......................................................,......... 61

Proposed Breakeven Experiment...................................................................... 62
Concluding Remarks ......................................................................................... 64

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Figures
Figure 1.1. An UIUC Spherical IEC .............................................................................. 1
Figure 1.2. Idealized Potential Structure Calculated by Hirsch for Monoenergetic
Ions With No Angular Momentum ......................................................... 6
Figure 1.3. Discharge Modes in Gridded Devices Identified by Miley ...................... 9
Figure 1.4. Photo of Star Mode Seen Through a Reaction Vessel Port Window ....... 9
Figure 1.5. Schematic of the UIUC RF Gun Injector for IEC Experiments .... .......... 11
Figure 1.6. RF Gun Attached to an IEC Chamber in the UIUC Laboratory .............. 12
Figure 1.7. Photo of Center Spot Formation ......................................................... 12
Figure 2.1. The "Historic" Early IEC Ion Injection Experiment ............................. 14
Figure 2.2. Neutron Rates Measured With the IEC: Neutron Rates Measured With
the IEC of Figure 2.1 Exceeded 10 9 DT n/s at 150 kV ......................... 14
Figure 2.3. Photograph of a STAR Mode Discharge ............................................... 16
Figure 2.4. IEC Landmine Detection Project at Kyoto University .......................... 17
Figure 2.5. Scheme for Landmine Detection Using a Hybrid Magnetron Type
IEC Neutron Source ............................................................................ 18
Figure 2.6. 3 He Ion Source for Use in 3 He - 3 He Fusion Rate Studies at the U Of
Wisconsin .................................................................................... ........ 19
Figure 3.1. Two Types of Cylindrical IECs ............................................................. 22
Figure 3.2. Jet Operational Mode in Experimental IEC Device and Jet Set-Up ...... 24
Figure 3.3. Illustration of the IEC Jet Thruster Experiment Showing Central
Grid and Two Jet Grids ....................................................................... 27
Figure 3.4. Dipole Reactor Propulsion Scheme ..................................................... 29
Figure 3.5. Dipole Magnetic Field and Layout of Devices ...................................... 30
Figure 3.6. Electron Density Vs. Dipole Magnet Field Strength at 25mtorr, 20mA 31
Figure 4.1. Cross Section of the Experimental Layout of the PFX-I Experiment .... 34
Figure 4.2. Detail of the Anode and the Ion Injection Port in PFX-I ..................... 35
Figure 4.3. Plot of the Q-Value ............................................................................. 37
Figure 4.4. Sketch of Two Opposite Limits of the Beam-Maxwellian Equilibrium .. 38
Figure 4.5. The Definition of the Double Well Depth ............................................. 39
Figure 4.6. The Definition of the Parallel and Perpendicular Velocities at the IEC
Cathode Grid ...................................................................................... 39
Figure 4.7. The Double Well Potential Calculated With IXL Code ......................... 40
Figure 4.8. Ion Density Profile for Potential Well ................................................. 40
Figure 4.9. The D-D Fusion Reaction Rate Versus Cathode Current ..................... 40
Figure 4.10. Plot of Growth Rate of Spherically Converging/Diverging Ion-Beam
Instability......................................................................................... 42
Figure 4.11. Diagram Showing Equipotential Surfaces of the IEC Cathode Grid .. 45
Figure 4.12. Results for Calculations for Ion Energy Distributions 1 st Pass ......... 45
Figure 5.1. Block Diagram of the IEC Pulsed Power System ................................. 49
Figure 5.2. Schematic (View From Top) of a Broad Coverage IEC Inspection

··············································••11••·······

System for Luggage Inspection
51
Figure 5.3. Further Illustration of the Integrated System for Airport Luggage
Inspection ........................................................................................... 53
Figure 5.4. Further Illustration of the Integrated .System for Ship Container
Inspection .......................................................................................... 53
Figure S.S. Image of a Fusion II Spaceship, a 750-MWe IEC Fusion-Powered
Manned Spacecraft With Ion Thruster Propulsion ............................. 54
Figure 5.6. Scale Schematic of Fusion Ship II, a 750-MWe IEC Fusion
Spacecraft .......................................................................................... 55
Figure 5.7. Illustration of a Three-Unit MCSA Device............................................ 57
Figure 5.8. Diagram of Belt-Cusp Fields and Particle Recirculation ............ ........... 57
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Figure 5.9. Axial Magnetic Field Strength Along Two-Unit MCSA Centerline ......... 58
Figure 5.10. Diagram of Direction Randomization (KAM Effect) Due to the
Magnetic Field Null Region in the IEC ............................................... 59
Figure 6.1. p- 11 B Fusion Cross Section Energy Requirements ............................... 62
Figure 6.2. IEC System With Radio Frequency Ion Gun ....................................... 63
Figure 6.3. Multiple Ion Gun Concept ................................................................... 63
Figure 6.4. Differentially Pumped RF-Driven Ion Gun ........................................... 63

Tables
Table 3.1. Estimated Performance Parameters or the IEC Ion Thruster ..... .......... 28
Table 4.1. Comparison of Analytical and Numerical Estimates of Q-Values in a
Beam-Dominated Solution -- for a SO-kV Square Well ......................... 32
Table 5.1. Three Kinds of Techniques Previously Used in Explosives Detection
System ................................................................................................. 48
Table 5.2. Comparison of IEC Design and Magnetic Fusion Design ....................... 56

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Inertial Electrostatic Confinement Fusion
Preface
This report is intended to provide the reader with an overview of the basics,
current experimental status, supporting theory, and potential applications of
inertial electrostatic confinement (IEC} fusion. Emphasis is placed on work in
these areas at the University of Illinois Urbana-Champaign, although some
other research is brought in.
The report shows that IEC is a unique approach to fusion in that it offers a
number of "spin-off" applications, such as a small neutron source for neutron
activation analysis on the route to fusion power. The report further shows that
IEC is one of the few potential fusion approaches that can potentially burn
aneutronic fuels like p- 11 B (hydrogen - Boron 11). In aneutronic fusion,
neutrons carry no more than 1% of the total released energy, greatly reducing
problems associated with neutron radiation. That ability, combined with its
simple mechanical structure and small size, make the IEC reactor, if achieved,
an ideal fusion power unit. Present experimental devices are four to five
orders of magnitude below breakeven (energy out/in = 1) energy gain for p11 B. However, it is argued that the ability to study the physics in very-small­
volume plasmas makes it possible to rapidly investigate scale-up to a power­
producing device. As an example, the report concludes with a conceptual
experiment proposed for demonstration of breakeven conditions for p- 11 B
using a hydrogen plasma simulation.
The author has purposely tried to avoid use of ,e quations in this report to
enhance readability and to stress concepts rather than analysis. However,
considerable analysis is provided in a number of the source references cited.

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Section I . IEC Background and Basics
Before considering detail, it is helpful to obtain a rough idea of how inertial electrostatic
confinement (IEC) fusion works.
Gas Feed
Line

High-Volugc

Power Supply

-l-

Figure 1.1. An UIUC Spherical IEC. The plasma discharge betwee·n the grid and vacuum wall creates an Ion
source that is extracted and directed towards the center by the high ly charged negative grid. A photograph of a
typica l IEC chamber is shown in the center. A photograph of the discharge through the view port shows the "Star
Mode" discharge where ion beams are created that pass t hrough the grid openings. This is Important for long run
times si nce ion bombardment of the grids, hence grid wire sputtering , is minimized.

For this purpose the experimental IEC device of Figure 1.1 is considered. As shown, this
"gridded" type IEC has a spherical mesh grid suspended on a high voltage feed-through
in the center of a metal vacuum vessel. The fusion "fuel", e.g. deuterium gas, is first
fed into the chamber originally prepared at high vacuum, e.g. 10-7 Torr. The fuel gas
brings the pressure up into the 10's of Torr region. Then the voltage on the grid is
raised into the many (-) kV range, creating a plasma discharge between the high
voltage grid and chamber wall (electrical ly grounded). The high negative voltage on the
grid serves to extract the ion from the plasma, accelerating them towards the center of
the grid where in principle they interact and fuse. In practice however, the scattering
cross section is larger than the fusion cross section. Thus many ions scatter without
reacting (fusing). Many "near misses" essentially pass straight through the center of the
plasma core and exit. This dominance of scattering over fusion reactions is the central
issue of all fusion confinement approaches, forcing use of strong confinement so the
ions have many passes and hence a good probabi lity of fusing before being lost from
the fusion reaction chamber. In the IEC multiple passes occur because the ions are
trapped in a potential "well" created by the buildup of positive charge due to the large
flow of the accelerated ions into a small "core" region in the center of the negative grid.
Viewed in another way, the ions extracted from the region between the grid and the
wall can scatter and pass back through the grid, but can only return to the same
potential surface they were born on. Thus they cannot reach the vessel wall, but
instead lose their kinetic energy, stop, and are accelerated by the grid potentia l back
into the center of the grid. This then provides many "recirculations" through the center
of the grid volume where they have a finite probability of fusing. If not for the existence

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of various loss channels such as hitting the grid, charge exchange, or up-scattering in
energy, the ions would be prematurely trapped in the potential well until they fused.
The conventional requirement for fusion confinement is given in terms of the
confinement parameter, nt, where n = the ion density and t is the confinement time.
Also the ion energy (or temperature T) must be in the 20 or more keV range assuming
D-T fuel. For breakeven, J. Lawson developed his famous "criterion" nt = 10 14 cm·3 -sec
at T > 15 keV for DT fusion. Here t = energy confinement time, sec; n = ion density,
cm ·3 and T = ion "temperature" or average energy.
The Lawson criterion is independent of the confinement method, but does depend on
the fuel via the selection of cross sections in the derivation. Magnetic confinement is
generally limited to n ~ 10 14 cm· 3 by pressure balance. Then a confinement time • of~
1 sec is required. For Inertia Confinement Fusion (ICF) or "laser fusion", compression of
targets can achieve n N 10 24 , so a confinement time of only 10-10 sec is need
(corresponding to the disassembly time of the compressed target). (For a general
review of energy breakeven requirement for D-T fusion and other fuels like D- 3 He and
p- 11 8, the reader referred to: G. Miley, Fusion Energy Conversion, American Nuclear
Society, La Grange, IL 1973).
Now consider the IEC. In principle, the ions focused on the center of the IEC can
achieve a density of n 10 16 , giving a required confinement time of 10- 2 sec for DT
fusion breakeven. This time can be restated in terms of the number of ion recirculations
in the IEC potential well by dividing the well diameter by the average velocity of the
recirculating ion. In later cases discussed in this report, this number is typically quite
large, usually N1000 recirculations. Achievement of this large number of recirculations
requires strong reduction of all of the loss channels noted earlier. Grid losses can be
reduced by STAR mode operation discussed later where the recirculating ions possess
beam -like trajectories passing through the center of the grid opening. The ideal,
however, is the elimination of the grid altogether which can be done via formation of
virtual potential structures, originally proposed by Farnsworth and discussed in
following sections. The temperature requirement also leads to a fundamental difference
in the IEC physics vs. other confinement approaches. (Note that "temperature" is not a
proper term here since it implies an equilibrium distribution while the IEC is far from
that with its beam-like ions. Thus, the reader should view "temperature" as meaning
average energy of the ions. In doing that, however, it is assumed that the ion energy
distribution is known so that averaging is possible). Most ions in the IEC are born near
the chamber wall so are accelerated to an energy close to the applied voltage on the
grid during the extraction process. A reasonable estimate is that the ions reaching the
fusion region in the center have an energy near 80 percent of the grid voltage on
average. Thus it becomes relatively easy to achieve the Lawson D-T requirement by
applying a voltage of~ 25 kV. In fact most IEC neutron sources discussed later operate
at voltages > 80 kV to get into an energy range giving a higher fusion cross section. In
sharp contrast, magnetic fusion devices struggle to obtain a temperature in the 10 keV
range since the entire plasma population must be heated (vs. direct ion acceler ation in
the IEC) due to the equilibrium distribution maintained in these plasmas. Another very
important point is that Lawson assumed that the ions and electrons were in thermal
equilibrium, at the same temperature, T. This is a reasonable approximation for
magnetic confinement, but not so for the IEC. In the latter, the electrons form a
"distorted" Maxwellian distribution at an effective temperature well below that of the

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beam-like ions. Since electron energy loss processes such as radiation emission are
serious, the Lawson temperature criterion must be modified for the IEC. A first rough
estimate is the Te/T1< 1/3 for DT. (Here T1and Te are the ion and electron
"temperatures", respectively). Control of th is ratio is a complex physics issue, involving
the relative ion and electron source rates and energies and the potential well structure.
In later sections use of p- 11 B (hydrogen-boron-11) fuel in the IEC is considered. This is
very attractive since it provides all charged particle reaction products, making this an
unique "aneutronic" system. Such a reactor represents a truly ideal system from an
environmental and energy sustainability perspective. However, for such fuels, t he
Larson criterion becomes much more demanding, increasing nT by two orders of
magnitude and T to 150 kV. Also, for the IEC, a Te/T1< 1/9 becomes essential. (Using
temperature ration is a very simplified representation. The radiation losses are quite
sensitive to deviation in the actual energy distribution of the ions and electrons . For
example, electron Bremsstrahlung emission primarily comes from the high energy "tail"
of the electron distribution while energy transfer with ions is dominated by the "foot" of
the electron distribution. At high powers, interactions in these regions can become quite
non -linear, depleting or "burning out 0 the local populations in these regions. This effect
causes energy losses to saturate, hence can be quite beneficial under some
circumstance . However, the phenomenon is complex to evaluate numerically, so little
has been reported on it for IECs to date). The very aggressive p- 11 B Lawson
requirement is employed in the design of the breakeven experiment of Section VI.
While this discussion of IEC physics has been greatly simplified, it hopefully provides
more insight into the basic concepts and issue before delving into more detail.

IEC BACKGROUND
Inertial Electrostatic Confinement (IEC) was conceived of by Philo Farnsworth, the
inventor of electronic television, as an approach to fusion power using electrostatic
fields for confinement (Reference 1.2). When he did this in 1955, the prime approaches
being pursued worldwide were magnetic confinement or inertial (laser compression of
targets) confinement. In fact, electrostatic confinement had been written off by most
scientists due to Earnshaw's theorem (Reference 1.4) which stated that plasma could
not be confined by electrostatic fields alone. That was simply an expression of the fact
that use of a biased plate to confine one species, say ions, would automatically attract
the opposite species, electrons, such that the whole plasma would transport to the
plate. Farnsworth seemed to intuitively understand that this theorem assumed steady­
state, so that if, as in IEC, the ions were dynamically moving and confined, they would
electrostatically confine the electrons. Farnsworth went further and realized that in a
spherical system virtual electrodes would form a high density plasma region if the
confined ions were focused at the center of the sphere (Reference 1.2).
While Hirsh worked with Farnsworth to demonstrate early experimental success with
IEC experiments (Reference 1.2), the concept passed from view as magnetic and
inertial confinement resea1rch exponentiated. Then in the late 1990s R. W. Bussard
revived the concept with the hybrid IEC magnetic approach (Reference 1.6- 1.7). In this
approach the electrons were confined in the magnetic field, forming a potentials trap for
ions. [Note the similarity to the original conceptual potential well discussed by Elmore,
et al. (Reference 1.1)]. Upon invitation by R.W. Bussard to join this effort, the author,
George Miley, undertook supporting experiments that were a variation of the original
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Hirsch approach, using electrostatic grids to form a t rap with ions that then brought
electrons in. He realized that this approach could result in a very attractive low level
neutron source for neutron activation applications, and began that development. Such
work was soon taken up in several other laboratories, including Los Alamos Nationa l
Laboratory (LANL), University of Wisconsin and Kyoto University. Meanwhile Bussard's
work continued with strong funding from the military. However, little was published or
known about this until 2008 when he made public appeals on "YouTube" to regain
funding stopped just when the experiment achieved a major success. Subsequ,ently,
funding resumed but R. W. Bussard passed away shortly thereafter due to a long battle
with cancer. His company and work were then taken over by R. Nebel who took leave
from LANL to undertake this new work. That effort is now in progress and represents
the largest IEC power oriented project in the US or elsewhere (but still modest with a
half dozen senior scientists involved). Meanwhile, laboratories elsewhere working on
IEC neutron sources have continued while the U of Wisconsin has added an IEC proton
source as an option using sim ilar technology. The labs, including the UIUC, have fusion
power as an ultimate goal, but must focus on their funded near-term "spin-off"
projects.
At this point the IEC still receives no funding from DOE which remains focused on the
Tokomak route to fusion power. Thus, with little funding, slow progress has been made
in answering the key question of whether or not the IEC can be developed for fusion
power. If it can, the device would be simpler and smaller than a Tokamak, malking it an
extremely attractive option. In addition, its beam - like reactions (highly non-Maxwellian)
make the IEC very well suited for burning alternate ("advanced") fuels like D- 3 He and
p- 11 B which are much more environmentally favorable than conventional DT fusion .
Unfortunately Tokomaks are not well equipped to go forward to such fuels.
On the other hand, the use of non-power-producing IECs for other applications, such as
sma ll neutron, proton, and x-ray sources, has been .amply demonstrated. Now, the
issue is how well and in what applications the IEC sources compete commercia lly with
other options such as accelerator target sources.
This report is intended to provide the reader with important insight into the physics and
technology of IECs relative to both power and neutron/proton/x-ray sources. With this
background, hopefu lly the reader can formulate an opinion about the potential for IEC
applications. Due to the limited funding for IEC research to date, much more has to be
done to actually demonstrate its application, especially for power production. Thus that
opinion must remain a personal one for the reader.
One other limitation of this report is that it largely provides details based on the
author's work on IECs over the last decade. Thus it will not do justice to the ongoing
work by others, notably at EMC 2 on the Bussard Polywell device or the advanced
gridded IEC neuron/proton so urce development work at the U of Wisconsin, Kyoto
University, and Tokyo Institute of Technology. Some insight into these efforts is given
in comments and in references supplied, but t he reader is encouraged to discuss that
work with those individuals directly.

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IEC BASICS
We begin by presenting the early very basic theoretical study by Elmore, Tuck, and
Watson (Reference 1.1). That addresses the key question of the fusion power density
obtainable with potential well confinement. One of their basic assumptions is that the
potential well is "dug" by electrons which trap ions. Certain added assumptions lead to
well depth, etc, and finally they conclude that the system is unstable for ion densities
sufficiently high that appreciable thermonuclear yield is expected. They qualify this
conclusion saying "admittedly, a more thorough investigation is required to obtain a
complete understanding of stability of this electrostatic device".
This result was quite negative for electron formation of potential wells, but left the
route possibly open since the subject "needed a more thorough investigation." Later,
for various reasons, R. W. Bussard still pursued this concept by introducing the High­
Energy Power Source (HEPS) Polywell device which uses a spherical simulated magnetic
field to stabili ze the potential well formed by electrons. This represents a "hybrid"
magnetic-lEC confinement system where electrons are confined by the magnetic fields,
forming the potential well wh ich "traps " ions. Apparently, Bussard's view was that this
added magnetic stabilization would overcome the earlier Elmore and Tuck criticism.
Subsequently, some of his reports used particle-in-cell simulations to support the view
that such a stabilized electron potential well would allow adequate density for attractive
fusion densities.
However, the next IEC experiments following the Elmore et al. analysis (prior to
Bussard's) were the Hirsch-Farnsworth experiments (Reference 1.2) which used ion (vs.
electron) injected traps (as does the present author's work). This selection was largely
driven by the desire to gain added stability by the large momentum of recirculating ions
that form the potential well. More about ion vs. electron injection routes will be covered
in later sections. Next, it is important to review the multiple well ("poissors" solution)
Farnsworth - Hirsch found for ion injected formation of potential wells in spherical
geometry. This is described in the paper by Hirsch (Reference 1.2). As seen from Figure
1.2, monoenergetic ions with angular momentum "drag in" electrons to create "onion
skin" like nested potential wells around the center of the sphere such that the ion
density goes to infinity in zero volume at the origin.

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1'10 ....

----- - - ' ' - - - . . ----- - - - - . - - - - - - - - - r - - - - - - - - - - - . . . , ,

10 ~ ~-

-

- --

- ----- - - - - - - - - - - - - - --+---- - --,t

.....-

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Figure 1.2. Idealized Potential Structure Calculated by Hirsch for Monoenergetic Ions With
No Angular Momentum. The nested virtual anodes and cathode s observed were originally termed
" poissors" by th e inventor, Philo Farnsworth .

This is a very striking result that enthused these researchers to push on with this
research. It, in effect, circumvents the Elmore et al. restriction by changing the
potential well physics fundamentally. Of course in practice, there will be a spread in
energy and angular momentum, so one would not expect more than a single potential
well (vs. the infinite poissors of Farnsworth) to form in practice. The questions
remaining then were (and still are): "How deep can such a well be in practice and how
high an ion density can be trapped in it?" Various studies followed to study these issues
more thoroughly using simulation codes. For examp le, Klevens and Black found in
Reference 1.3 that: "A model of an electrostatic confinement device with ion injection
has been developed which provides strong correlation between theory and experiment.
The ion density profile was determined in position velocity throughout the two
concentric grids by considering the processes of cha r ge transfer and grid capture. A
shallow-well approximation was incorporated in the model by assuming that ions
encountering charge transfer in the inner grid region were accelerated up to a
maximum of 5 percent of the applied grid voltage, and that the velocity of beam ions
was constant in this region. Distribution functions in total energy and angular energy
were developed for both ions and electrons. The ion distribution function consisted of
three parts: a beam created at the anode and accelerated by the applied cathode
voltage; a low -energy group produced by charge transfer near the cathode or in the
center; and a intermediate-energy group resu lting from charge-transfer reaction
between anode and cathode. For each group the angular energy was assumed uniform
up to a maximum value, which was different for each energy group. The electrons were
assumed to be isotropic in velocity space, and to be uniformly distributed in total
energy in the potential well in which they are trapped . The distribution functions were
substituted into Poisson's equation and potential and density profiles for various
6

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experimental parameters were obtained. The parameters which were varied include the
background pressure, the ratio of electron to ion circulating currents, the applied
potential difference between the grids, and the inner grid variables such as measured
current, transparency, and construction error. When estimates of each of these
parameters which simulate the ion injection mode experiment were inserted into the
model, the resulting potential profile exhibited no more than a shallow potential well.
This result is consistent with that of beam defection measurements in the ion injection
mode experiments. In spite of these assumptions, the model is extremely valuable in
determining the relative (sensitivity of the potential well profile and depth to effects of
many of the system parameters.
It has been found that for the "medium" level of ion currents under discussion, the
most critical factors which inhibit deep well formation are inadequate spherical focusing
and charge neutralization. The focusing is determined to a great extent by the degree
to which the grids are spherical potential surfaces. The grid must approach a spherical
shape within a few percent before other factors such as grid transparency and
background pressure play an important role. However, as the current is increased, the
requirements for sphericity are somewhat relaxed. For a grid construction error of less
than 5 percent, increasing the grid transparency and decreasing the pressure will also
lead to significant improvement in well depth."

These results were somewhat encouraging. However, they showed that grid
deformation could be very harmful. This deserves several comments. First, the present
author (G. Miley) later showed that design of grids with larger openings provided the
STAR mode where ion beams passed through the center of the openings, avoid ing grid
collisions and making sphericity of the grid itself less important. This is important for
small neutron/proton source type devices . However, the assumption of grids fails to
address the question of how a grid could survive in a power reactor or if they could be
eliminated to use a potent ial well with virtual electrode formation. The use of grids
cannot be completely ruled out for power reactors. Magnetic field protection techniques,
active cooling, etc. are conceivable.
Another question relates to the role of background gas in the IEC. The point is this:
When Miley moved to simplify the device for small neutron sources, he used the
discharge between the grid and vessel to form the ions needed for acceleration and
fusion. This inherently forces use of a modest background neutral gas pressure of the
fuel (typically deuterium) inside the reaction vessel. That in turn results in ion reactions
with the background gas becoming a dominant process in these IECs . Such interaction
includes fusion itself, scattering, charge exchange, etc. This greatly changes the plasma
physics of the IEC as opposed to the ideal of a potential well with "zero" background
pressure. Some key differences in the physics of such IECs were brought out by Tim
Thomson in his experimental study described in Reference 1.4. He noted that: "In
present gridded systems, convergence is not important since beam -target fusion
reaction dominate the reactivity of these devices, as evidenced by the linear scaling of
reactivity of these devices, as evidenced by the linear scaling of reactivity with the
cathode current. In fact, convergence may reduce the reactivity by forming a virtual
anode that limits the centr al ion density. However, this space charge effect can be
overcome by proper introduction of electrons. Good convergence is required to achieve
optimal beam -beam reactivity scaling for the application that require higher fusion
reaction rates, and the importance of symmetry in determining convergence places a
7

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constraint on any Spherical !EC device planned for these applications. The observed
loss of convergence with decreasing pressure and increasing current makes achieving
significant beam -beam scaling far less favorable ."
In add ition to the issue of beam -background fusion dominating in the gridded systems
at higher pressures, Thomson pointed out the importance of energetic ions undergoing
charge exchange and being lost from the system. This work and others on the small
gridded systems at that time showed several important problems which are best
understood by considering beam-background vs. beam -beam fusion scaling. The former
scales with density and pressure as nb • nok <av>
lilb • p <av> while beam-beam
fusion goes as nb2 <av > .

~

Here: nb = beam ions per cm 3 ; nbk= background atoms per cm 3 ; p = background
pressure; <av> is the fusion reactivity averaged over the appropriate beam ­
background (or beam-beam) distribution functions.

If ions are produced as done in most small gridded experiments by electron ionization
collisions with neutral gas during a plasma discharge between the grid and vacuum
vessel wall, reduction of background gas pressure will also reduce the ion source,
reducing the reaction rate. Thus, it becomes apparent that to get the favorable beam­
beam scaling needed to go into the power reactor regime, ions must be produced
externally while the main reaction chamber is keep at very low background pressure to
avoid charge exchange losses. Indeed, without explaining that this was the reason,
Hirsch used external ion '\guns" in his early experiment at Farnsworth labs. The present
author (G. Miley), however, went back to the internal discharge ion source technique to
simplify the device for portable neutron source appli,cations. Power devices will need to
go back to external production of some type however. Again, this issue will be
addressed further later.
While earlier workers sought small grid open ings designed to provide uniform ion flows
for good core plasma convergence (stressed in the earlier papers already noted), Miley
disclosed in a paper in Reference 1.5 that the STAR mode could be produced with wider
grid openings. In fact, Miley noted that three key modes can be formed in gridded IECs
depending on the pressure and grid openings. These are described as:
"Glow discharge operation of the IECGD is categorized by three distinct discharge
"modes": Star, Central Spot, and Halo (illustrated in Figure 1.3). These names are quite
descriptive of the visual appearances of the visible light emitted from the discharges. All
three modes are reproducible and stable; each is associated with a different potential
well structure, hence neutron production rate. The star mode was used extensively in
recent experiments. It is distinguished by microchannels or "spokes" radiating outward
from a bright center spot (Figure 1.4). As verified by magnetic deflection experiments,
the spokes are primarily composed of ion beams aligned so that they pass through the
center of the openings delineated by the grid-wires. This mode is very efficient for
neutron production, since the large effective grid transparency allows numerous passes
of ions through the center spot before being intercepted by the grid or being ion by
charge exchange. The Star mode is typically obtained at lower operating pressures
(<10 mTorr) and higher voltages (>30 kV), using a carefully formed grid with good
sphericity and high transparency (>95 percent). The halo (or "jet") mode occurs when
one of the grid openings is slightly enlarged compared to the others."
8

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I

,

'I

,

I

I

I

\

I

•

I

I

I

... ___ ,.,. ;

,

I
I

Figure 1.3. Discharge Modes in
Gridded Devices Identified by
Miley

Figure 1.4. Photo of Star Mode Seen Through a Reaction Vessel
Port Window

In summary, the basic IEC approach is to create a potential well through electrostatic
confinement of one of the plasma species in a dynamic (inertial) configuration.
"Inertial" effects associated with dynamic motion of the confined species are essential
to avoid plasma losses predicted for systems by Earnshaw (as noted earlier). The two
primary approaches can be termed, "ion injected" or "electron injected", the "injected"
species being the one form in g the potential well. In order to mainta in the well, the
second species brought in with the injected one must not completely neutralize the
plasma, i.e., the IEC plasma is inherently "quasi-neutral". This well then provides
trapping and convergence of the ion "streaming" towards the center of the trap region,
forming a dense fusing plasma there. For a power reactor the objective is to obtain ion
beam-beam collisions in this central core. For neutron/proton production satisfactory
reaction rates can come from beam background collisions. However, this sca ling with
injected current would require excessive input power for a practical power-producing
unit. Thus beam-beam scaling of the reaction rate as the current squared (or higher
powers as noted earlier may be possible due to nonlinear effects) is essential. The
vision of a power reactor seeks a "zero" background pressure, thus generally involves
an external ion source with acceleration into the trap at ultra low pressure to obtain
beam -beam collisions. As described earlier, this changes the details of the physics just
discussed for an ideal "zero" background pressure device. The issue of whether the trap
should be formed by ion injection or by "digging a well" with electrons remains open,
but involves stability and reaction volume (focusing) optimization issues. Since the
discussion to here has been largely on gridded devices, we next briefly review some
other approaches: the Bussard HEPS concept, the Barnes Nebel Penn ing trap, the Nebel
POPS device, and the Miley ion injected device.

9

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Bussard HEPS (or Polywell) Concept
In Bussard's Polywell IEC, a spherical magnetic field termed a "Polywell" is
approximately obtained with a multi-pole cusp magnetic field (Reference 1.6). More
about the theory of flows in this configuration is given In the paper by N. Krall
(Reference 1. 7), and some stability issues are addressed in the Wang and Kral I paper of
Reference 1.8. One of the key physics revolves around electron losses from the poles in
the cusp field. Krall and Bussard argue that a plasma "waffle-ball" effect causes the loss
cone angle to be reduced due to the high pressure developed in the IEC plasma. The
issue still needs further experimenta l verification. The Polywell approach is very
important and it is currently pursued by R. Nebel's EMC 2 company in Santa Fe with
significant DOD funding. More insights will be provided throughout this report, but the
reader is encouraged to study the reports/articles, as already explained earlier, since
the present report is directed more at "ion injected" type devices studies at UIUC.

Barnes Nebel Penning Trap
The Penning trap concept described in Reference 1.9 is explained by Barnes et al. as:
"The Penning Fusion (PF) device uses a unique plasma confinement principle. In PF, a
nonneutral electron plasma is confined in a modified Penning trap by a combination of
applied magnetostatic and electrostatic fields. The e!lectron space charge, in turn,
electrostatically confines a minority, unmagnetized ion species. To apply such a system
to fusion energy production, it is necessary to raise the applied voltages (producing the
confining electrostatic field) to the order of 100 kV or greater. Even with such a high
potential, in a practically sized system, the electron density (and to a greater degree
the ion density) falls short of that required to give reasonab le fusion reactivity . Thus,
intrinsic to PF being an interesting concept is the idea of ion focusing, either in space or
time, or some other means of enhancing ion reactivity. In this way the reactivity may
be greatly enhanced over that available with the background density. Penning Fusion is
strongly related to the IEC, but it attempts to address two limitations of the IEC. First,
following the Bussard -Krall Polywell theory (Reference 1.6, 1. 7) the grid is replaced by
an electron cloud, which forms a virtual cathode. In this way, ion-grid collisions and
associated limitations (such as secondary electron emission from the grid and grid
heating) are avoided. Second, high rates of ion-ion collisions, which limit the
theoretically achievable fusion ga in Q (fusion power/input power) to around unity are
avoided to some extent with this type of well. However, issues of electron loss and cone
losses, radiation damage of the magnets and cooling, and the ability to circumvent the
Elmore et al. density limit remain as questions.

Nebel POPS Device
Theoretical studies by Barnes and Nebel (Reference 1.10) show that a small interna l
oscillating ion cloud may undergo a self-simi lar collapse in a harmonic osci llator
potential formed by a uniform electron background. This then forms a dynamic IEC
device, but with a quite different ion distribution factor vs. the "conventiona l" beam -li ke
one. A key issue for this concept is how much plasma compression can be achieved by
the POPS (Periodically Oscillating Plasma Sphere) oscillations. Recent work has shown
that by properly programming the distribution function of the injected electrons it is
possible to significantly improve the space dynamic charge neutralization and the
plasma compression. Reference 1.10 extends that pr evious work in a systematic
fashion by developing a formalism that determines the required velocity distribution of
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the injected electrons so space charge neutralizatio111 can be achieved. This formalism is
then included as a boundary condition in a gridless particle code. Results indicate that
although the formalism works well during the early phases of compression, when the
compression gets large the solution bifurcates and becomes unphysical. Subsequent
experiments on POPS at Lawrence Livermore National Laboratory (LLNL) were
encouraging, but have not been continued at a high level of effort due to key staff
leaving for EMC 2 . Thus, the practicality of this concept remains an open question which
deserves more research.

Miley's "Ion Injected" Device
The key to developing a IEC power device is to use external ion "guns" to form and
inj ect Ions into the spherical IEC chamber. This eliminates the need for a grid and
differential pumping between the gun and chamber allows the high vacuum needed in
the chamber. (Ion injection by external guns was originally used by Hirsch as already
noted. Also, more recently other labs, e.g. the University of Wisconsin and University of
Kyoto/Tokyo Institute of Technology, have started gun injection work. Some of that is
described later in this report) . The ion formation is done in the high pressure gun
discharge region outside of the chamber. Miley at UIUC (see Sections IV and VI) has
been studying such a system, both theoretically and experimentally. The theoretical
studies confirm that such an IEC plasma can exist stably and has sufficient confinement
time for aneutronic fusion. This assumes, however, very precise control is maintained
over the energy and angular momentum of injected ions and a balanced supply of
electrons is provided. A radio-frequency (RF) ion injector (or "gun") capable of such
operation has already been developed . A sketch of th is design is shown in Figure 1.5.

magnetic focusing lens
coaxial copper resonator: hollow cylindrical

l

stainless steel flange
ceramic (insulator)

upper plasma stream: floating

negative potential

Q~

D2 gas feed

n beam~t,,J;,l;~==;:;===l:~l=t'A'Ft,VtJil/1t:tl't~l=l:~l4..,._t:t,:==;=f=i=r"iF=;;:=l

positive wall,
part of vacuum chamber ~

l

j
helical antenna

! glass tube

magnetic differential coils

RF generator
lower plasma stream: floating
Figure 1.5. Schematic of the UIUC RF Gun Injector for IEC Experiments

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In this RF gun, a graded index magnetic field is used to increase the ionization
efficiency. A key component is the magnetic focusing lens at the extraction port. This
allows very efficient differential pumping between the high pressure gun chamber and
the low pressure IEC chamber. It also provides some control of the angular vellocity of
entering ions.
The UIUC RF ion-injector is shown attached to an IEC chamber in Figure 1.6, and a
photograph of the focal spot achieved with injection from this single injector is shown in
Figure 1. 7. Note that ion scattering off of the center dense plasma "core" causes
noticeable (but "faint") recirculating ion beams observed in the photograph of the
discharge. With additional injectors, the recirculation pattern should become quite
symmetrical about the center. These studies did include differential pumping so that
number of recircu lating passes, ~. by an ion was very low, roughly 2. The injected ion
current, I, was about 50 mA. Still, based on measurements of neutrons emitted using
deuterium fue l, the Q (fusion energy gain/energy in) was remarkable for such a small
device, order of 10·6 . Based on these results, an aggressive p- 11 B breakeven
experiment using this type of IEC is discussed in Section VI.

Figure 1.6. RF Gun Attached to an IEC Chamber in
the UIUC Laboratory

Figure 1.7. Photo of Center Spot Formation. The
main beam observed is a direct path along the injector
angle. Other faint lig ht channels indicate beams for
scattering of the central core region.

CLOSING REMARKS
As seen, a wealth of information has been developed in studies of gridded IEC devices.
However, the beam- background fusion used in these devices involves important
differences in physics compared to what is needed for future beam -beam IEC reactors.
Most notable is the need to maintain an extremely low background pressure to prevent
interactions with background neutrals. Further, physica l grids are subject to damage at
high power levels. As pointed out, some studies show grids can survive at modest
powers. But, for aggressive power units such as the p- 11 B plant of Section VI, they
must be replaced with virtual electrode surfaces creating a deep potential well for ion
confinement. Upscattering out of the well must be minimized while electron

12

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temperatures are suppressed. These issues will be discussed further in Section IV on
theory and Section VI on a proposed breakeven experiment.
REFERENCES
1.1
W. Elmore, J. Tuck, and K. Watson, "On the Inertial-Electrostatic Confinement of
a Plasma", Phys. Fluids, vol. 2, No. 3, ( 1959) pp . 239-246 .
1.2
R.L. Hirsch, "Inertial-electrostatic confinement of ionized fusion gases," J. of Appl.
Phys., vol. 38, (1967) pp. 4522-4534.
1.3
W. M. Black and E. H. Klevans, "Theory of Potential -Well Formation in an
Electrostatic Confinement Device", J. of Appl. Phys., Vol. 45, No. 6 (1974) pp. 25022511.
1.4
T. A. Thorson, R. D. Durst, R. J. Fonck, and L. P. Wainwright, "Convergence
Electrostatic Potential, and Density Measurements in a Spherical Conver gent Ion
Focus", Phys. Plasmas, vol. 4, no. 1, (1997) pp. 4 -5.
1.5
G.H. Miley, et al., "Inertial -electrostatic confinement neutron proton source,"
Third International Conference on Dense 2-pinches, AIP Conference Proceedings 299,
(1993) pp. 675-689.
1.6
R.W. Bussard, "Some Physics Considerations of Magnetic Inertial - Electrostatic
Confinement : A New Concept for Spherical Converging - Flow Fusion", Fusion
Technology, vol. 19, no. 2, (1991) pp. 273-293.
1.7
N. A. Krall, "The Polywell™: A Spherically Convergent Ion Focus Concept", Fusion
Technology, vol. 22, nol, (1992) pp . 42-49.
1.8
S. K. Wong and N. A. Krall, "A Nonlocal Theory of Counterstreaming Ion
Instability", Phys. Fluids B, vol. 5, no.6, (1993) pp. 1706- 1714.
1.9
D. C. Barnes, M. M. Schauer, K. R. Umstadter, L. Chacon, and G. H. Miley,
"Electron equilibrium and confinement in a modified Penning trap and its application
to Penning fusion", Phys. Plasmas, vol. 7, no. 5, May (2000).
1.10 R. A. Nebel, D. C. Barnes, Fusion Technology 38, 28 (1998).
1.11 D. C. Barnes, R. A. Nebel, "Stable, thermal equilibrium, large -amplitude,
spherical plasma oscillations in electrostatic confinement devices", Physics of
Plasmas 5, 2498 (1998).
1.12 J. Park, R. A. Nebel, S. Stange, S. K. Murali, "Periodically osci llating plasma
sphere", Physics of Plasmas 12, (2005) 056315.
13
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Section II . Select Experiments
In this section some select experiments are briefly reviewed with the main focus on ion­
injected IECs. Beginn ing with the early Hirsch gun injected IEC experiments that are
important both historically and from a physics perspective. Other experiments aimed at
"spin -off" applications are then covered, followed by a discussion of severa l recent gun
injection type studies. Readers interested in more detail should consult references.
In Reference 2.1, Robert Hirsh disclosed experiment al results with very high D-T
neutron rates from an ion-injected IEC shown if Figure 2.1.
Note that six ion "guns" were used to create a low energy ion beam that entered the
chamber and was trapped via electrostatic structures to form the potential well
structure desired for IEC operation. However, differential pumping was not used so
beam -background and charge -exchange collision must have still played a significant
role in this experiment. Still Hirsch obtained "record" neutron rates for DT fusion shown
in Figure 2.2.
ROBERT L , HIRSCH
,010
r,e/ ·--,
'
~
[g
:
HIGH VOL.TAG[
DOME
CONU,INING
AUXIL A,tY
POWER
SUPPI. IES
0
u
w
109
ll)
....
~
0
Q::
t-
~
tHGH VOLTAGE
INSUL. ATOA
.
co.I
108
I-
:,
0.
6
107
I:J
.z
106
..
AP PUEO POT
Figure 2.1. The "Historic" Early IEC Ion Injection
Experiment of R. Hirsch Working With Philo
Farnsworth
Tl~L -
V
Figure 2 .2. Neutron Rates Measured With the IEC of
Figure 2,1 Exceeded 10 9 OT n/s at 150 kV. For
perspective, note tha t, as discussed later, UIUC IEC gridded
devices routin ely produce 10 8 DD n/s at "' 80 kV. This is
sl ightly above Hirsch's result but uses higher ion currents.
The key point about this remarkable result is that the neutron production rates are well
about that predicted by simple beam-background fusion reactions, implying that benefit
14
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was obtained from recirculation beam-beam reactions in a potential well such as in
Figure 2.1 (but without multiple structures). Indeed, to further confirm the existence of
a potential well, Hirsch did both collimated neutron and gamma measures. As shown in
the paper, he found structure for both consistent with well formation. One possible
explanation is that the ion-electron densities obtained were high enough to "burn out"
(completely ioni ze) the background neutrals in the potential well. There is no direct
evidence to support this view however.
These important results have never been fully explained. Attempts to reproduce his
experiments were done by Gardner and co-workers at Brigham Young University
(Reference 2.1) who borrowed the original device used by Hirsch. However, despite
many months of effort, the neutron production they obtained was significantly lower
than that reported by Hirsch. They attributed this problem to a failure to regain the gun
alignment necessary to have a highly converged plasma "core" in the center of the
device. A major hurdle to this appears to have been that no provision was made to
allow precision alignment of the gun ions entering the device (although the
investigators did not mention this explicitly). Later when Miley reinitiated gun
experiments, his first gun design followed many of the design elements used by Hirsch,
but incorporated electrostatic beam steering. This worked well, but the design was
eve ntually discarded to move to RF guns with much higher beam currents. In addition,
the gun desig n of Figure 1.5 uses a magnetic nozzle for reducing the exiting beam
diameter and to allow strong differential pumping (not used in the prior Hirsch
experiments).
It should be stressed again here that the terms "injector" and "gun" are misleading.
The objective is to simply "flow" low energy ions into the device such that they are then
accelerated to fusion energies by either the grid or the virtual electrode structure. Thus,
a loss of "excess" energy after injection is needed trap the ion, i.e. prevent it from
simply passing through the potential well and hitting the opposite wall. A biased
reflector on the opposite wall can be introduced to help prevent this, but this only works
well if the entering ions have little excess energy. To further understand this problem,
the reader is advised to study the design of the Hirsch chamber of Reference 2.2 which
uses an auxiliary biased grid ("reflector") near the wall. Indeed the issue of how to best
introduce ions into t he potential well so t hat t heir energy fa lls below that required to
escape the well is a key for proper design of the IEC. In addition to designs to cause an
initial ion energy loss to "drop" them into the potential well, designs with ion sourced
" imbedded" in t he well such that ions are born trapped are discussed later.

Gridded devices for near-term applications such as neutron activation analysis (NAA) do
not rely on virtual well potential traps. Rather, the negative bias of the grid forms a
potential trap, and ions are born within the potential trap by ionization collisions in the
internal plasma discharge.
Note that the electron injected case faces the same problem of getting ions into the
potential trap. The approach used with the Polywell employs an embedded ion source
plus relies on "burn out" densities to eliminate neutrals. Success with this technique,
after many problems, was the key that lead to the "breakthrough" reported by R. W.
Bussard (as noted earlier) just before he passed away.

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In 1997 Miley and his group published
an IEEE paper that summarized their
internal ion injected grid experiments
(Reference 2.3). (As noted earlier "ion
injected" has been used to define the
species forming a potential well. It is not
to be confused with external "gun"
injection where ions form the well, but
are introduced from an external source).
It discusses the discharge physics and
plasma characteristics for various modes
of operation. It explains how the STAR
mode is created by the defocusing
properties concave inward (towards the
center core) in open grid structures.
Indeed the concept is somewhat anti­
intuitive since one might hope for
focusing "optics", but this is not possible
Figure 2.3. Photograph of a STAR mode discharge.
The "vane'' type grid shown Is only one of a number of
in these configurations. (Indeed various
large opening grids designed and used for STAR mode
multi-grid approaches have been studied
operation of the neutron source type IEC. This parti cular
with the objective of improving beam
design (but with variations) has been used by the UIUC,
Daimler-Chrysler and Kyoto University. It Is rugged,
optics for reflection of ions, hence
shows
little sputtering and has proven very efficient for
recirculation. See for example, Reference
neutron production . Materials used vary from stainless
2.4. When ions pass through the
steel to Mo.
concave potential, all but those in the
exact center of the curved surface are deflected and lost. The centered ions pass
through to the opposite side and go through the grid opening, then are reflected and
repeat this trajectory. Subsequently ionization events along this path cause a rapid
increase in the recirculating current through the center of the grid openings. This then
produces the beautiful STAR mode discharge shown in Figure 2.3.
The use of a pulsed power supply represents a very important way to study the physics
of high current IECs without employing expensive, very large power supplies and also
avoiding the need for strong cooling to remove the waste heat. The key physics point is
that the beam-beam fusion rate scales as the ion current squared. Most steady state
experiments employ 100s of mA, while pulsing peak values of many amps are possible.
By selecting the pulse width to match or exceed the ion confinement time, typically
order of ms in present devices, a quasi equilibrium is established during the pulse. This
allows study of "equivalent" steady-state physics du ring the pulse.
Miley's device in Reference 2.6 used a Marx bank technology to provide peak currents
of l0's of amps with a ~0.1 sec width and a low repetition rate (selected to minimize
cooling requirements and also reduce bank recharging requirements).
Another important technology regarding the IEC vessel pumping was developed in the
mid - 1990s by staff from Miley's group working at the Idaho National Environmental and
Engineering Laboratory (!NEEL), Idaho with Robert A. Anderl (Reference 2.5). This work
substituted a metallic hybrid getter for the external pumping on the IEC chamber. With
this arrangement, the deuterium is absorbed in the getter material while the vapor
pressure, hence chamber background pressure, is controlled by regulation of t he getter

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temperature. With proper selection of the size and type of getter material, plus a good
temperature feedback control, this arrangement was found to work exceedingly well.
This allows a "sealed" IEC unit and removes the bulky pumps. Such an arrangement is
essential for small mobile neutron sources. Daimler-Chrysler licensed use of this IEC
neutron technology through the UIUC and used this approach for their NAA quality
control units.
The general strategy employed by Daimler-Chrysler and others is to send sea led units
into the field for NAA application. The impurity buildup in the chamber gas eventually
causes the performance to deteriorate. At that point, the unit is returned to the
originating "factory" or originating laboratory for refueling by pump ing down and, if
necessary, replacing the getter. As it occurred, Daimler-Chrysler uses such units in
Germany on some of their ore delivery belts for NAA inspection of ore composition. In
this role they directly replaced Cf-252 neutron sources, allowing on -off operation,
simpler licensing, and lower costs. Their plan to distribute commercial units externally
for sale did not materialize, however, due to company financial problem.
Another important example of the use of a compact IEC neutron source is the work at
Kyoto University, Japan (Reference 2. 7). These investigators used a crane-type device
which was design for mounting on a crane as shown in Figure 2.4 for land mine
detection.

Project of Landmine Detection

R of LM OetectJOn
• Diagnostics~
Kyo o-U . T T, 'f\J uu-U.
• Tomography;
I
-bay E rgy

•

R O of compact IEC

• C /pulse IEC;
Kyo'o-U, K
• CW/Pu se power
supp!y ; Tl.,.
The prQJect is suppooed by Japan
Science and echnology gency

Figure 2.4. IEC Landmine Detection Project at Kyoto University

17
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The basic principle of this detection method, shown in Figure 2.5, is common to most
NAA but is now specialized for detecting the basic elements in the land mines. Key
design considerations are the source strength required, the neutron energy desired (i.e.
D-D vs D-T fusion) and the type and location of neutron and x-ray detectors (from
Reference 2.7).
How to Detect Landmine
Neutron yield of -1 08 is required
y~ay
detector
neutron
source
I
-~-- ~ ~
,...,
·i-~~ ~
ta111•
- - -~...
•
., \
~-
Alo •
I
3
•
-
I
~
T explosive
,
Ato, 1 r io of
3
I~ I
iv
y ray en i
N(n, y) • • • 10.83 MeV y ray n i
fix d
ion
ion
Figure 2.5. Scheme for Landmine Detection Using a Hybrid Magnetron Type IEC Neutron
Source (From Reference 2.7)
The IEC developed for this work used a magnetron ion generation technique to improve
the neutron production efficiency (Reference 2.6). A built-in magnetron discharge ion
source was installed in the IEC. With the magnetron discharge, ions are produced in the
vicinity of the vacuum chamber (anode) at negative electric potential. Therefore, the
ions produced are expected to have nearly full energy corresponding to the applied
voltage to the IEC cathode but slightly smaller energy than the anode potential. This
prevents them from hitting the anode of the opposite side improving both fusion
reaction rate and ion recirculation life. (Note that thiis approach is yet another way to
address the problem of preventing ions created externally from escaping after entering
the potential well. The technique here is to use the internal source to create the ions at
a potential level less than the height of the potential! well).
In add ition to an internal source, the magnetron can produce ample ion current to
maintain the discharge under low-pressure conditions . Ions generated in the ion source
are attracted by the IEC central cathode because of its high negative electric potential.
Therefore, the Kyoto investigators expected that a higher applied voltage would
increase the extraction current, giving a higher IEC cathode current. However, it was
18
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found that there is an optimum voltage in terms of a maximum IEC cathode current.
Ions supplied by the magnetron ion source are essential to maintain the hybrid (glow
and magnetron) discharge under low gas pressure conditions. The reason for this
voltage lim it is not clear. Unfortunately this effect limits the cathode voltage despite the
need for high voltage for neutron production near the peak energy of the fusiorn cross
section. Thus this issue deserves more study to optimize use of a hybrid magnetron
source IEC operation. Still the design worked reasonably well for the initial mine
detection experiments. Unfortunately, the project was terminated prematurely due to
financial constraints, so information on optimization possibility remains incomplete.
The design of the ion source also depends on the ion species being injected. Sources
described thus far have focused on deuterium or, in some cases on tritium. However,
workers at the University of Wisconsin (U. Wisc.) have had a great interest in 3 He
reactions (both D- 3 He and 3 He- 3 He), so have developed a gun specialized to 3 He ion
production (Reference 2.8). To maximize the ion current, a Helicon source was selected
since Helicons are well known for production of very high density plasmas from which
high ion currents can be extracted. This source is illustrated in Figure 2.6 and discussed
in Reference 2.8. They state that this source has produced steady-state ion currents of
10 mA into IEC systems with background gas pressures as low as 200 µtorr.

Figure 2.6. 3 He Ion Source for Use in 3 He- 3 He Fusion Rate Studies at the U of Wisconsin

These developments are aimed at observation of the 3 He- 3 He reaction in an IEC device.
Scoping calculations of beam-background fusion rates predict that a 3 He- 3 He reaction
product spectrum should be distinctly observable in an IEC operating below 200 kV.
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This capability can provide valuable data regarding 3 He fusion cross sections at "low"
energies with better counting statistics than accelerator measurements. It appears that
good progress has been made in this direction.

CLOSING COMMENTS
The experiments selected for this section are far from exhaustive. The main
concentration here is on ion-injected IECs such as studied at the UIUC, although
various electron injected devices such as the "Polywell" are mentioned. The
experiments were selected then to explain some issues and status relative to gridded
devices for near-term applications such as neutron sources and also to address some
issues such as ion injection related to future fusion power units. The latter issues
revolve around how to create deep potential wells in the IEC and trap the reacting ions
in the well while excluding neutral gas atoms. The use of external ion sources with
differential pumping then becomes a key approach for production of ions while keeping
ultra low background pressure in the reacting chamber. This is the approach used at
the UIUC. However, introduction of the source into the configuration such that the ions
are born at potentials below the well depth is another possibility as shown by the hybrid
magnetron source work in Japan. Another point noted is the advantage of using pulsed
operation to obtain high peak ion currents to take advantage of the ion density squared
scaling for beam-beam reactions.

REFERENCES
2.1

R. L. Hirsch, "Inertial -Electrostatic Confinement of Ionized Fusion Gases," J. Appl.
Physics 38, no.11, (1967) pp. 4522-4534.

2.2

A. L. Gardner, "Studies of Charged-Particle Distributions in an Electrostatic
Confinement System, "U. S. Atomic Energy Commission Final Report N. C00 -21 80-7,
Washington, D. C. (1974 ).

2.3

G. H. Miley, Y. Gu, J. M. DeMorea, R. A. Stubbers, T. A. Hochberg, J. H. Nadler,
and R. A. Anderl, "Discharge Characteristics of the Spherical Inertial Electrostatic
Confinement (IEC) Device," IEEE Transactions Plasma Science, Vol. 24, No. 4, (1997)
pp, 733-739.

2.4

T.J. McGuire and R.J. Sedwick, "Improving IEC Particle Confinement Times Using
Multiple Grids" 7th US -Ja pan IEC Workshop, Los Alamos National Laboratory, NM,
March 14-16 (2005).

2.5

R.A. Anderl, J.K. Hartwell, J.H. Nadler, J.M. DeMora, R.A. Stubbers, and G.H .
Miley, "Development of an IEC Neutron Source for NDE," 16th Symposium on Fusion
Engineering, eds. G.H. Miley and C.M. Elliott, IEEE Conf. Proc. 95CH35852, IEEE,
Piscataway, NJ, (1996) pp. 1482-1485.

2.6

Y. Gu, M. Williams, R. Stubbers, and G. Miley, "Pulsed Operation of Spherical
Inertial-Electrostatic Confinement Device", Fusion Technology, Vol. 30, no. 3, (1996)
pp, 1342-1 346 .

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2.7

T. Takamatsu, T. Kyunai, S. Ogawa, K. Masuda, H. Toku, and K. Yoshikawa, "A
Magnetron Discharge Ion Source for an Inertial Electrostatic Confinement Fusion
Device" 7th U.S. -Japan IEC Workshop , Los Alamos National Laboratory, NM, March
14-16, (2005).

2.8

G. R. Piefer, J. F. Santarius, R. P. Ashley, G.L. Kulcinski "Progress in the
Development of a 3 He Ion Source for IEC Fusion", 7th U.S. -Japan IEC Workshop, Los
Alamos National Laboratory,, NM, March 14-16, (2005).

Section III. Other Geometries
A un iq ue feature of the IEC is the ability to vary its geometry to adapt to a number of
important near term and future applications. Here we consider cylindrical IEC
geometries, the IEC Jet extraction geometry, dipole assisted, and magnetically-coupled
IEC unit which add flexibility for use in some power applications. Other important
configurations, which are quasi-spherical, include the magnetic assisted HEPS (Polywell)
configuration, the Penning trap IEC, and the POPS oscillating IEC. These concepts are
discussed briefly elsewhere in this report so will not be included here.

CYLINDRICAL IECS
The prime alternate geometry studied for IECs is cylindrical. While originally developed
at the UIUC, the configuration has spread to other labs including the University of
Wisconsin, Kyoto University, and the Tokyo Institute of Technology. The objective is to
obtain a dense core region extending along the axis of the cylinder. This is especially
important for neutron sources since it offers a very long source that can be used for
broad area coverage of large objects such as container boxes. Other conventional
sources would require multiple "ganged" sources to do the same. A downside however,
is the high power input required for such configurations. Thus the advantage of source
length must be weighed against the alternative of moving a smaller point source over
the surface of interest.
It is not clear that the cylinder is useful for scaling to a power reactor. It can be viewed
as a 2-D version of the spherical unit. As such, the beam convergence (compression) is
limited to lower values, hence lower core denslties (an important effect for beam -beam
fusion desired for power reactors, but less so for beam-background reactions used in
most current neutron sources).

Two types of cylindrical sources (References 3.1 - 3.5), shown schematically in Figure
3.1, have been studied - a gridded type which is essentially the spherical unit
converted into a cylinder, and a quite different hollow cathode design. The gridded
design was a natural variation of the original Farnsworth device and was first studied
experimentally in the 1970s by T. Dolan at the UIUC who used laser diagnostics with a
noble gas discharge to study density-temperature and species profiles. The hollow
cathode design was later proposed by G. Miley as an attempt to retain the long axial
reaction region but do away with grids.

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a)

b)

~
I

Reflector

/

Figure 3.1. Two Types of Cylindrical IECs. The hollow cathode on the left (often termed the C-Device) and the
2-D gridded version on the right, often just called "the" cylindrical IEC.

The hollow cathode "C-Device" of Figure 3.1 has an insulated vacuum chamber with an
alternating series of hollow cylindrical cathodes and anodes spaced along a common
longitudinal axis. Biased end plates serve as charged particles "reflectors". The anodes,
cathodes and end plates are biased to steady state and/or pulsed voltages, depending
on the operational mode. This configuration is used to initiate a plasma discharge,
resulting in electrostatic confinement of fusion fuel ions in both the axial and radial
directions (Reference 3.4). Present operation produces about 10 7 n/s (D- D) steady
state while for pulsed output 10 9 n/s (D-D) is obtained.
In "the" cylindrical version 2-D IEC (Figure 3.1), a cylindrical cathode grid is placed with
its axis concentric with the axis of the surrounding vacuum vessel. This is then, in
effect, a 2-D version of the spherical IEC. It operates by convergence of ions cr eated
between the grid and wall onto a small volume along the axis, hence has sometimes
been called the "Radia l Converging IEC (RC-IEC)". As already noted, the basic physics
of this version was originally studied in pioneering work by T. Dolan in 1970. However,
the concept lay dormant until revised and upgraded several decades later with
improved grid designs for neutron production by UIUC workers .

ELECTRICALLY- DRIVEN IEC JET THRUSTER
The use of an !EC design for space propulsion was originally proposed by R.W. Bussard
(Reference 3.6) . (His concept was for a high thrust scram jet device. While very
attractive, this concept (and Miley et al.'s "Spaceship I & II concepts discussed later)
are for far-term use. Here we discuss near-term electronically driven IEC designed for
space app lications. In this case, electrical power would come from a solar panel.
The IEC jet thruster is intended as an ultra-maneuverable space thruster for satellite
and small probe thrust operations. The IEC Jet design potential offers a unique
capability to cover a wide range of powers (few Watts to Kilowatts) with good efficiency
while providing a plasma jet that can start with a large diameter but be narrowed
directionally to focus on targets. Th e IEC thruster uses a spherica l configuration,
wherein ions are generated and accelerated towards the center of a spherical vacuum
chamber. A virtual cathode forms in t he high-density central core region, and combined
with a locally distorted cathode grid potential field, extracts accelerated ions into an
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intense quasi-neutral ion jet. The configuration, low gas leakage, and good heat
removal make it possible to scale the design to either low powers or high powers,
covering a range of interest for present small satellites on to future medium and large
satellites. In addition to maneuverable thrusting, the jet channel extraction technique
enables directing and focusing the plasma stream down on an asteroid or other object
for interrogation of it. Analysis of the plasma emission spectra would provide an
identification of the materials and surface features of the object. With further
development the IEC system potentially offers an att ractive fusion power source.
Another advantage of the IEC jet thruster is that it provides a step towards a future p11 B IEC power source and/or thruster for satellite operations. This possibility is also
briefly discussed here.

Relation to Other Prior Thrusters
NASA and other laboratories have worked toward developing advanced Hall Thrusters
for future satellite applications. Such thrusters, however, do not scale well to lower
powers for small satell ites, nor are exhaust plasma modifications possible to provide
fast maneuverability. The IEC-jet thruster appears uniquely able to address both issues.
Conventional plasma thrusters such as the Hall thruster have undergone much more
experimental study than the IEC-jet thruster. However, the simplicity of t he IEC-Jet
thruster design and its thermal scalability makes it feasible to quickly develop and test,
making the lack of data base less of a liability.
In the jet thruster concept the plasma target at the center of the chamber, created by
the intersection of the multiple ion beams, serves to deflect ions into the escaping jet
plasma. The resulting virtual anode, in combination with curved potential lines created
by t he cathode grid diverts ions, forming a strong plasma jet. This is channeled out
through an enlarged hole and guide structure in the grid (Figure 3.2). This design
promises a good efficiency and thrust while providing a low weight, and due to the very
open accelerator grid structure, a very long lifetime. Thus it provides a good thruster
for basic satellite operations and with the added jet co ntro l/focusing also provides
maneuverability.
In addition the IEC jet offers two added features that increase its potential effectiveness
for probing various space objects. The fact that the IEC jet can be controlled to form
over multiple areas around the sphere would allow tlhe platform to maneuver itself close
to a target and then simply open a second jet offset 180 degrees from the propulsive
one. The second jet would serve the integration purpose of the platform without having
to expend time or additional resources such as fuel to reorient itself to direct t he plume
at the target. Other current systems, such as Hall thrusters, would first have to position
itself close to the target, and then reorient such that the exhaust plume is properly
oriented. Thus, the IEC jet thruster would not be subject to expending the resources of
time and fuel that other platforms require. Another option for the IEC jet thruster is to
operate as a pu lsed device. This becomes especially important when considering how
long it may take to disable a defensive target (the platform's impulse time to disable).
The use of an intense pulsed jet could disable the target before it has time to maneuver
or apply defensive layers. As discussed later, the basic IEC has been operated
experimentally in a pulsed mode using a capacitive power unit. However, to date,
formation of the jet has only been studied under steady-state operation.

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An added long-term potential advantage of developing the !EC-jet thruster is that the
concept can eventually be extended to an ultra high impulse fusion power/propulsion
unit. Indeed, the thruster concept arose from work on a fusion based IEC (Reference
3.5). To move to fusion power, however, would require advances such as discussed for
terrestrial IEC power plants as Section VI.

JET Extraction From a Spherical IEC
A plasma jet can be extracted from the gridded spherical IEC by enlarging a grid
opening. This forms the basis for various exciting applications including a jet thruster
and various plasma processing uses such as gasification of municipal wastes and future
materials recycle. Here, to illustrate this asymmetrical geometry, we briefly consider
the IEC jet thruster for use in satellite operations and maneuvering (Reference· 3.6).

Description of the IEC let Thruster
For conventional star mode operation, such as discussed earlier, the IEC grid is
designed to be highly symmetric so that the microchlannel beams are also symmetric,
providing good convergence. However, experiments have demonstrated that enlarging
one of the grid openings distorts the potential surfaces. This results in the creation of a
very intense, tightly coupled space -charge-neutralized ion jet directed outward from the
central core plasma region (see Figure 3.2) (Reference 3. 7), and it is this mode of
operation (star with jet) that would be employed for the proposed thruster.

Xenon

Electron
Guide

+ IOOV

~

Enmtters

(a)

(b)

Figure 3.2. (a) Jet Operational Mode in Experimental IEC Device and (b) Jet Set-up

The jet formation has been explained theoretically (Reference 3.5) in terms of the large
distortion of the potential surface at the enlarged grid opening. The local gradient
initiates electron flow that in turn drags ions out across the surface. The result is the
formation of the intense space charge neutralized ion beam (or "plasma jet") at that
location. Such operation has been routinely obtained in laboratory IEC devices under
steady-state operation, with the plasma jet being maintained for hours. The power
carried by the jet has been demonstrated by heating a target plate placed in its path.
Over half of the energy imparted to the ions by the accelerating grid is effectively
funneled into the jet, and no major losses of ions to the vacuum chamber wall or grid
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are observed. For example, with the device of Figure 3.2b, the input power is about 2
kW with over 1.5 kW being carried out by the jet flow. Consequently, this configuration
provides a way to efficiently convert the energy stored by accelerated ions in the
symmetric microchannels into a directed beam or jet.

Comparison to a Conventional " Plasma " Thruster
The IEC thruster can be viewed as transforming a conventional ion thruster into a
spherical form. In the IEC thruster of Figure 3.2b, ions are produced in the gas
discharge region through the injection and oscillation of electrons about a guide grid
that is held to a slightly positive potential. A second grid extracts ions from the
discharge region and accelerates them towards the center of the device. This type of
ion formation in the IEC has been studied extensively by workers at Nambe Tech, a
"spin-off" company by LANL workers (Reference 3.8). In the case of a planar thruster,
ions are also formed in a discharge region. However, instead of a guide grid, a
magnetic field is used to contain electrons, providing a long path to maximize ionization
collisions. The ion source regions are basically the same, then, for both IEC and planar
ion thrusters, and the extraction/acceleration provided by the grids and the magnetic
fields play equivalent roles in the respective devices. However, the IEC jet thrnster
concept must extract a net thrust from the spherical! configuration. That is where the
two concepts (planar vs. IEC) diverge, and the star-jet operational mode becomes
essential. The micro-channel formation in the STAR mode acts to maintain and store
the accelerated ions until they are diverted directionally and escape out through the
plasma jet opening . The opening is biased, enabling the control of the potential gradient
between the grid and the opening, hence allowing for increased control of the intensity
of the plasma jet.
The maintenance of STAR mode is then fundamenta I to the success of !EC jet
operation. Both experimental data and particle-in-cell (PIC) code simulations show that
the energetic ions can be maintained in microchannels for hundreds of passes. The jet
opening allows escape into it in less than ten passes. Consequently, a majority of the
ions, >95 percent, escape at full energy. Thus, the star-jet provides an amazingly good
method to both store and direct the energetic ions.
Overall, the IEC configuration offers distinct advantages. First, the grid structures used
in the IEC thruster are much more open than those used in the conventional planar
thruster designs. When combined with microchannel ion focusing, this prevents ion -grid
collisions, greatly reducing grid erosion and significantly increasing the thruster lifetime.
Second, transforming the device into a sphere makes the overall unit more compact per
unit-ion -source volume, implying a weight reduction advantage. Third, unwanted
neutral propellant gas leakage would be reduced, due to the smaller net opening
required by the high-density jet, vs. a conventional planar thruster having multiple grid
openings. This advantage becomes especially important for thruster applications.
Assuming that the IEC thruster efficiency would be comparable to that of the planar Ion
thruster design, based on these operational improvements, the !EC thruster then offers
significant advantages. [Note that an alternate !EC thruster concept is described later in
the section on J. Khachan's work. It too looks very promising.]

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let Extraction
To obtain thrust from an IEC device, a valley or trough must be created in the
electrostatic potential, and a hole must be physically cut into the ground sphere. This
allows high-speed ions to escape in the form of a plasma jet as described in the
preceding section. Ions are generated near the ground potential with the aid of electron
emitters and additional grids. A central spherical electrical grid accelerates ions to the
core region. A cylindrical "channel" grid with the same electrostatic potential as the
central spherical grid creates a passage through which ions can escape to the outside.
Thus this trough in the electrostatic potential profile across the centerline (thrust axis)
of the IEC thruster is a unique feature compared to the linear profile for a planar
device.
The ion beams finally exit through an opening in the ground sphere of the IEC device .
The channel grid must be well insulated from the ground potential to prevent short­
circuiting or arc-over; thus a separate insulated feed-through cable maintains the
negative potential on the inner spherical and channel grids. Makeup propellant gas is
fed into the ionization regiion throug h needle-valve-controlled tubing located around the
chamber wall. Ionization of the propellant uses the electron emitter-guide grid design
described earlier. The inner grid serves to both extract and accelerate ions, forming the
microchannels. To control neutralization of the plasma jet, additional electron emitters
are attached close to the jet discharge hole . A combination of electron emission rates
and jet grid bias can be used to control the beam space charge, hence the growth of
focusing of the beam during propagation. In present experiments, however, the
electron rate is fixed at a suitable value while the jet grid bias is varied.
Control of the jet diameter and focus is obtained in two ways: first, the channel grid will
be separately hinged with a small servo motor such that its axis can be moved over a
volume defined by a 10 percent cone angle; second, the grid bias can be varied over a
range up to the chamber potential to provide focus control over the jet flow. A large
negative grid bias will cause a narrow focus while small potential values will allow the
plasma to expand giving a broad cross section beam.

Experimental let Design and Performance
Figure 3.3 shows the thruster experiment components. It is also of interest to •consider
the typical dimensions involved to illustrate the compactness of the unit. The
experimental studies use an existing spherical IEC chamber of ~30-cm with a 1-cm
diameter port on one side of it for beam extraction. (This is somewhat larger than
envisioned for application to micro-satellites). An 8-cm diameter spherical electrical
tungsten or tantalum wire grid, having a geometric transparency of ~90 percent, will
mount inside the chamber. A ~1-cm diameter hole will be cut into the side of the wire
grid, and this hole will be aligned with the hole in the chamber wall and connected to it
by a 2-cm diameter cylindrical guide grid. The insulator covering the grounded wall
must be of sufficient size to prevent arc-over from the ground to the cylindrical grid.
The inner electrical grids are connected to a 500-kV de power supply through the
insulated feed -through cable. A positively-charged outer grid with a variable voltage of
10-100 Vis mounted on a swivel connector at the outside of the beam extraction
port of the chamber, in combination with four electron emitters, generates ions. The
choice for the voltages on the outer grids is flex ible, so long as a sufficient ion

~

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generation rate is achieved. The net accelerating voltage must be kept within the order
of 1 kV to ensure that the resultant exhaust velocity is in the range of 30,000 m/s (lsp
< 3000 seconds). The mission for a communications satellite is optimi zed with a
specific impulse in th is range. A higher specific impulse would reduce propellant
requirements, but the power requirements and the mass associated with power source
components would increase correspondingly. Xenon is bled into the chamber through
holes at appropriate locations around the wall of the vessel.

Figure 3.3. Illustration of the IEC Jet Thruster Experiment Showing Central Grid and
Two Jet Grids, Opposite to Each Other With On- Off Action

Table 3.1 lists the estimated performance parameters for an IEC electrostatic i-on
thruster based on experimental data available to date plus extrapolation of other ion
thruster data. The IEC device will have higher densities and temperatures in the central
core plasma than a conventiona l thruster, but the increased losses due to
Bremsstrahlung radiation are still negligible when compared to the other power losses.
Thermal radiation losses should be comparable to that in conventional thrusters. The
energy expenditure per ion for conventional ion thrusters and is 300 eV per ion. The
energy expenditure per ion for the IEC device has not been established experimentally,
but was estimated in Table 3.1 from simulation studies.

~

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Table 3.1. Estimated Performance Parameters or the IEC Ion Thruster

Parameter
Propellant
Molecular Weight ( arnu)
Specific Impulse (s)
Thrust (m.N)
Jet Power (W)
Net accelerating Potential (V)
Beam Current (mA)
Power Loss to Grid (W)
Power Loss to Bresstrahlung
Radiation (W)
Power lo s to Ionization of
Propellan t (W)
Input Power (W)
Thruster Efficiency(%)

IEC Ion Thruster
Xenon
131.3
3000
34
500
600
832
:S50

<1
200-250
750-800
62-68

In summary, the power efficiency of the IEC thruster appears to be competitive to
existing ion thrusters. What are the advantages then? These were outlined earlier and
includ e a more compact design, large heat rejection area, an exhaust jet closer to
quasi-neutrality, reduced neutral propellant leakage, and reduced grid erosion. Thus,
the mass of the IEC jet thruster system can potentially be reduced compared to a high­
power Hall-type t hruster and also its lifetime can be increased significantly. In this
overall context, then, the IEC thruster potentially offers an important improvement in
performance for high power thruster applications.

Scale- up to p- 11 B IEC Space Power Unit/Thruster
The electrically driven IEC jet thruster provides an important data base for a next step
p- 11 B IEC jet thruster. Jumping to p- 11 B for this application may appear overly
ambitious. However, neutronless fusion seems essential in a sma ll space thruster to
avoid excessive weight from shielding of electronics . Considerable experience with
fusing plasmas in IECs has been gained through development of IEC DD neutron
sources. These devices operate with
80- keV D-ion beams using the non -Maxwellian
character of the IEC. This important characteristic makes use of p- 11 B a realistic goal.
In fact, operation with circulating ion energies at the desired 150 keV energy for p- 11 B
has already been achieved at the UIUC and several other laboratories working on IECs.
The issue then is how to achieve adequate confinement times . The approach being
pursued at UIUC is the formation of deep potential wells with angular ion injection using
a differentially-pumped RF ion gun, as discussed in later sections. A proposed
experiment to demonstrate p- 11 B physics is discussed in Section VI.

~

In summary, the extraction of a jet plasma from a gridded IEC opens the way to a
number of added plasma applications for the IEC. This present discussion is intended to
identify an orderly progress of IEC applications in commercial space power, starting
with an electrically driven IEC thruster to a self-powered IEC p- 11 B unit. The attractive
characteristics of the electrically driven device, namely light weight, low mainten ance,
28

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low fuel leakage and extreme maneuverability make it a near-term competitor with
other devices such as Hall thrusters for future commercial thruster applications in the
multi -kW range . The extension to a p- 11 B self-powered unit would resolve many
problems anticipated as larger power requirements develop. It would be extendable to
large power units needed for eventual fast deep space propulsion. Much more r esearch
and development is required to ensure that step in a timely fashion, however.

The Dipole Assisted IEC (DaIEC)
The dipole assisted IEC DaIEC is similar to the IEC concept discussed above except a
dipole magnet is located in the center of two hemispherical grids (Reference 3. 9). The
DaIEC was first proposed by G. Miley at the UIUC and has been under investigation
there. This concept is closely related to the levitated dipole reactor (Reference 3.10)
but is much simpler, being smaller and not requiring levitation. It also differs
considerably in the physics of the associate plasma confinement. Two ion sources inject
40-keV deuterium and helium-3 ion beams toward the center of the dipole magnet. The
magnetic field will compress the ion beams by trapping ions along the magnetic field
lines; therefore, they fuse within the dipole magnet. The products of the D- 3 He fusion
reaction are 14. 7-MeV protons and 4-MeV alpha particles. These can be used for direct
charged particle propulsion or direct conversion to electricity (or both - propulsion and
station keeping). A schematic of the setup is shown in Figure 3.4.

[:E]

G:EJ

[8J
Stabilizing coil
Figure 3.4. Dipole Reactor Propulsion Scheme

Those ions that exit toward the right in Figure 3.4 are trapped by the magnetic field
produced by the stabilizing coil and are exhausted to produce thrust. Since the
magnetic field does not close at the nozzle but is open, protons and particles are not
required to be neutralized. This configuration of the magnetic field in the DaIEC system
reduces the mechanical components. A neutralizer (electron injection into the exhaust)
will be required in this system so as to avoid possible charging up at nozzle.
29

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There are several advantages of the dipole-assisted IEC. By applying the desired
voltage to the cathode grid, high-energy ions are easily obtained, hence plasma heating
is straightforward. Indeed in this case fusion is dom inated by beam-beam (non­
Maxwellian reactions). Also, the dipole magnet at the center of the device produces
field lines that trap ions and compresses them within the inner radius of the dipole.
Thus, a very high ion density can be achieved leading to high reaction rates via beam­
beam fusion. Biasing the dipole magnet to the same potential as the cathode grid
solves the problem of space charge build-up due to the high ion density at the center of
DaIEC.

DAIEC EXPERIMENTS
The purpose of current experiments is to investigate the focusing effect of a dipole
magnetic field in a spherical IEC. In particular, the primary goal is to measure the
increase in plasma densities achieved by a dipole-assisted IEC. In theory, the
introduction of a current ring or dipole coil into the base configuration would focus
particles into the center of the dipole, increasing the plasma density at the core of the
device. A schematic of the current experimental setup is shown in Figure 3.5.

Figure 3.5. (a) Dipole Magnetic Field and (b) Layout of Devices

This experiment employs two split spherical grids as shown in Figure 3.5. Ions are
created in the discharge between these grids and the vessel wall. They are extracted
and accelerated by the grid potential so they pass through the dipole field of Figure
3.5a . Experiments have confirmed that an order of magnitude density increase (vs. no
dipole present) can be achieved in the center region. A double Langmuir probe is
inserted at various positions throughout the center region.
The use of a bias on the dipole magnet structure to control space charge bui ld-up was
also studied. In an ion-injected IEC partial space-charge neutralization at the core
30

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region is essential to decrease the center potential and consequently allow incoming
beam Ions to penetrate into the core region . In the DaIEC, it is possible to assist this
effect by an external control of the centerline potential by applying a bias voltage to the
dipole coil. In the UIUC experiment this was done by applying a voltage to a small
copper ring inserted in the core of the dipole magnet . The increase in the electron
density with a small center line bias applied was about 30 percent while the appl ied
voltage to the dipole structure was ~3 percent of the discharge voltage. The present
setup prevented the use of larger applied voltages which could further improve
neutralization, but these results still demonstrate the basic concept of dipole focusing
with coil bias control.
Figure 3.6 shows how the electron density changes with increasing magnetic field
measured at various coil biases in the current experiment. It is found that electron
density increases about 17 times more than that of non - magnetic field measurement.
So far, this increase is very close to our theoretical estimations.
Electron density vs B-field 25mTon---. l.OOE+lO
('f")
I
~
·-a
ell
d
(1)
"O
l.OOE+09
d
g
~
~
l.OOE+08
0
100
B-field strength (Gauss )
200
300
I-
400
500
G-ound
---+- ~ =- JOOV - - - Vc=+ l OOV
600
700
I
800
Figure 3.6. Electron Density vs. Dipole Magnet Field Strength at 25mTorr, 20mA
The next section turns to some very interesting diagnostic studies of IEC devices,
including a jet thruster configuration, done by Joe Khachan's group at the University of
Sydney, Australia.
KHACHAN 'S STUDIES AT U OF SYDNEY
Joe Khachan's IEC research at the University of Sydney has stressed studies of optical
emission spectroscopy of gridded IEC devices (Reference 3.11, 3.12). Using Doppler
spectroscopy of the Hydrogen Ha line they have shown that the micro -channels in an
IEC discharge operating in the units at tens of mTorr pressure range (for voltages less
than 30 kV) are mostly composed of molecular ions with approximately 20 percent
atom ic hydrogen. They have also developed a spectr oscopic model based on collisional
radioactive modeling to spectroscopically measure ion densities, electron energies and
31
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predict fusion rates. This latter achievement makes spectroscopic measurements
simpler to carry out since hydrogen can be used to predict these fusion rates, which
excludes the radiation hazard in a laboratory situation where shielding is not possible.
In follow-on work, they simplified the modeling of charge exchange with an analytical
approach to charge exchange modeling based on Markov chain theory.

Table 4.1. Comparison of Analytical and Numerical Estimates of Q-values in
a beam-Dominated Solution -- for a SO-kV Square Well.

Analytical

BAFP
(Chacon}

With co-moving ions

Q ~ 0.21 - Ref. 4.1, Nevins

-

Without co-moving ions

Q ~ 1.3 - Ref. 4.2 , Chacon

Q~l

The movement of neutrals moving away from the cathode seen from the spectroscopic
measurements was also confirmed by carrying out a dusty plasma measurement to
show charged micron sized insulating spheres (dust) experience a force away from the
cathode centre (Reference 3.13). The explanation of this was attributed to a local
potential maximum established at the center of the cathode, which accelerates ions at
that point away from the center. Due to charge exchange, ions become neutrals and
are able to leave along the microchannels out to the anode. The ion drag force on the
dust moved the dust particles away from the center. Based on this observation,
Khachan has engineered the collimated beam of exiting neutrals from the cathode to
make a simple electric propulsion thruster where a unidirectional micro-channel
emerges from a conical cathode (Reference 3.14 ). He claims that the specific impulse of
the thruster and its efficiency greatly exceed existing electric propulsion thrusters. Note
that this concept, while having some similarities, differs in some details from Miley's jet
thrusts describe earlier.

CONCLUDING REMARKS
The primary focus of this section has been on two key alternate geometries: The
cylindrical and jet IECs. Both provide unique capabilities for applications using IEC
sources and IEC space thruster, respective ly. These uses, however, require competition
in commercial markets with other options. This IEC technology is just now emerging, so
its success in commercialization has yet to be established. More about such applications
is discussed in Section V.
In conclusion, the DaIEC is a very interesting alternate IEC configuration. However, it
has received little experimental study to date, so much more needs to be done to fully
evaluate its potential.

REFERENCES
3.1 B. Bromley, L. Chacon, and G. Miley, "Approximate Modeling of Cylindrical Inertial
Electrostatic Confinement, (IEC) Fusion Neutron Generator," Proc. 16 th International
32

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Conference on Numerical Simulation of Plasmas, UCLA Dept. Physics, Los Angeles,
CA, Santa Barbara, CA, Febl0 - 12 (1998) pp. 191 - 192.
3.2

G. Miley, T. Bauer, J. Nadler, H. Hora, "Converging Beam Neutron Source for
Driving a Sub-critical Fission Reactor", Proceedings, !CONE 8, CD/Track 8, 8500 Apr
2-6 Baltimore, MD, (2000)pp. 1-8.

3.3

R. A. Stubbers, H. J. Kim, and G. H. Miley, "Two-Dimensional Modeling of a
Radically -Convergent Cylindrical Inertial Electrostatic Confinement, (IEC) Fusion
Device," 5th US/Japan Workshop, IEC Fusion, Madison WI, Oct 9-10 (2002).

3.4

G. H. Miley, R. A. Stubbers, and H. Momota, "Advances in Cylindrical IEC Neutron
Source Design for Driven Sub-Critical Operation," to be published in the proceedings
of ICONE 11 : 11 th International Conference on Nuclear Engineering, Nuclear Energy­
Dawn of a New Era, !CONE 11, Tokyo, Japan, Apr 20-23 (2003).

3.5

G. H. Miley, J. Javedani. R. Nebel, J. Nadler, Y. Gu, A. Satsangi and P. Heck, "An
Inertial Electrostatic Confinement Neutron/Proton Source," Third Internationa l
Conference on Dense .Z pinches, editors. Haines, Malcolm, Knight, and Andrew, AIP
Conference Proceedings 299, AIP Press, (1994), pp. 675 -689.

3.6

R. W. Bussard, "Fusion as Electric Propulsion," J. of Propulsion, Vol. 6, No. 5,
(1990), pp . 567-574.

3.7

Y. Gu, .B., Nadler, J.H., Satsangi, A.J., Javedani, J.B., Miley, G.H., Nebel. R.A. and
Barnes, D.C., "Physics and Effects of Grid - Electric Field Perturbation on Spherica l
Electrostatic-Inertial Confinement Fusion". 21st EPS Conference on Controlled
Fusion and Plasma Physics, editors Joffrin, E., Platz, P. and Stott, P.E., Montpellier,
France, Vol. 436, (1994) .

3.8

D. C. Barnes, R. A. Nebel, F. L. Ribe, G. H. Miley, J. Javedani, Y. Gu. and A.
Satsangi, "Inertia l Electrostatic Confinement Experiments at Low Working Pressure,"
Prepared under U.S. Army contract DAAK70-93-003 8 by the Nambe Tech Corp.,
NTTR- 101, Santa Fe, NM (1994).

G. H. Miley, H. Momota, P.J. Shrestha, R. Thomas, and Y. Takeyama, " Space
Propu lsion Based on Dipole Assisted IEC System", AIP Conference Proceedings,
Albuquerque, New Mexico STAIF, vol. 813, (2006)pp. 1240-1248.

3.9

3.10 A. Hasegawa, L. Chen, and M.E. Mauel, "A D- 3He Fusion Reactor Based on a
Dipole Magnetic Field," Nuclear Fusion, Vol. 30, Sept. (1990), pp. 2405.
3.11 J. Khachan, S. Coll is," Measurements of ion energy distributions by Doppler shift
spectroscopy in an inertial electrostatic confinement device", Phys. of Plasmas, vol.
8, no. 4 (2001) pp. 1299- 1304.
3.12 0. Shrier, J. Khachan, S. Bosi, M. Fitzgera ld, N. Evans, "Diverging ion motion in
an inertial electrostatic confinement discharge", Phys. of Plasmas, vol. 13, no. 1,
(2006). Art. No. 012703 .

33

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3.13 J. Khachan, A. Samarian, "Dust diagnostics on an inertial electrostatic
confinement discharge", Phys. Letters A, vol. 363, no. 4 (2007) pp . 297-301.
3.14 L. Blackhall, J. Khachan, "A simple electric thruster based on ion charge
exchange", vol. 40, No. 8, (2007) pp. 2491-2494.

Section IV. IEC Theory
e emitter
Early basic IEC theory was briefly
Onion-shaped
described in Section I. Section IV will
-W0 kV
conductor
now turn to some more recent studies
starting with an early study by Bill
Nevins (Reference 4.1) that has caused
concern in the community about the
suitability of the IEC for a fusion power
reactor. Nevins did a semi analytic
analysis where IEC systems are
predicated including a non -equ ilibrium
ion distribution function. Coulomb
collisions between ions cause this
distribution to relax to a Maxwellian on
the ion-ion collisional time scale. His
analysis suggests that the input power
required to prevent th is relaxation, thus
Reflector
Superconducting
maintaining the IEC configuration for
magnet
times beyond the ion-ion collisional time -Wo kV· few V
Figure 4.1 . Cross Section of the Experimental Layout
scale, is greater than the fusion power
of the PFX-1 Experiment. The emitter -electron source,
produced. Thus, he concluded that IEC
onion-shaped anode and refl ector form an axial
systems show little promise for the
electrostatic well for electron axial confinement. Rad ial
development of commercial electric
confinement is provided by the axi al magnetic field. The
reflector is biased slightly more negative than the emitter
power plants. Nevin's analysis appears
to avoid electron losses to th e refl ector.
to be very thorough, however, as
discussed next, it suffers from severa l
key, but subtle assumptions that may force the pessimistic results.

B

ov

Later, to further explore issues raised by Nevins, Lu is Chacon, doing his thesis with G.
Miley, decided to use a Fokker Plank model for analysis of the IEC so that some of the
questionable assumptions used by Nevins could be relaxed. This study, presented in
Reference 4.2, specifically dealt with a Penning-type IEC due to interest in the Penning
trap experiment at LANL. The experimental device, PFX-I is illustrated in Figure 4.2
while the reactor-like configuration modeled by Chacon is shown in Figure 4.2. It should
be stressed however, that the conclusions still apply in principle to the ion injected IEC
since the issues involve the potential well trapping common to both. The Penning trap
and the ion injected devices differ in how the well is formed and stabilized, but the
physics of trapped plasma confinement is the same. Namely, the time scale for
collisional degradation of the beam-like ion distribution function is crucial since short
times (as the fusion time) would prevent a power reactor. Nevins addressed this issue
by calculating collisional relaxation rates from a beam-like, monoenergetic ion
population, absolutely confined in a square potential well. From his analysis, Nevins
concluded that the IEC will thermalize and lose ion focusing before enough fusion
34

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events take place. Accordingly, he predicted that the Q-value (defined the ratio of
fusion power out to ion input power) of an IEC device operating with a 50/50 percent
deuterium - tritium (D -T) mixture would be "'0.21 for a 50-kV square well. This
conclusion would rule out the possibility of a fusion reactor, but would leave open the
development of driven neutron sources. However, this analysis contains several
questionable assumptions. For example, a tightly focused monoenergetic ion beam is in
fact a pessimistic scenario, because different co - moving ion species (such as D and T
with the same energy) result in a finite speed difference, thus fostering ion -ion
collisions and the degradation of the ion distribution function. It would be more realistic
to consider that, in a square well, friction between species would homogenize the speed
within the ion beam after some time, making the speed difference infinitesimal. Th is
line of argument was pursued earlier by Barnes et al., (Reference 4.3) who found Q"'
1. 3 for the same system.

ION INJECTION PORT
Ion extraction grid

Ion divertor
Electron divcrtor

I
Pseudo-spherical
well for ion confinement

Anode wall (ground)
Figure 4.2. Detail of the Anode and the Ion Injection Port in PFX-I (not to scale). Ion
and electron divertors are ind icated, as well as the Eo and Emax equ ipotential lines that define the
ion co nfinement region. The Eo contour line determines the region of absolute io n confinement.

In Chacon's work, a bounce-averaged Fokker-Planck (BAFP) model was employed to
obtain steady-state solutions for the ion distribution function and to calculate associated
fusion energy gains (Q-values) in a variety of operating conditions. These is done in
terms of source and sink strengths, ion injection energies, well depths, and electrostatic
potential shapes. Thus, the limiting assumptions by Nevins- namely that ions are
confined in a square potential well, and that their distribution is tightly focused and
monoenergetic, are relaxed. When these restrictive assumptions are removed, it is
found that la rge energy gains (Qs of hundreds) for beam-like solutions in square wells
35

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are possible in Penning IEC devices provided that: 1) The electrostatic well is deep
enough (Eo > 100 kV); 2) Ion confinement time is long enough ( 8 > 0.01); and 3) Ion
source strength is moderate (Smax < 300). Another important effect which enters these
calculation concerns is relative to ion motion in the well. To emphasize the difference
between Q values studies from Nevins and Chacon are summarized in Table 4.1. (Here,
the BAFP calculation was done for off-optimal conditions (vs. the BAFP obtained for
beam -like cases) to compare with the Nevins' calculations) .
Further, ca lculated Q-val ues from the BAFP simulation by Chacon for the beam-like
cases are about five to ten times larger than those obtained in by Nevins in Reference
4.1. This added inconsistency can be traced back to the different treatment of the D
and T ion species in the two ca lculations. Thus, while BAFP treats both species as one
with average mass, Nevins treats both species separately but assumes they follow the
same monoenergetic distribution function. This results in a finite velocity difference
between species that boosts collisionality, rendering smaller Q-values. The
inconsistency in the Q-value disappears when BAFP is compared against theoretical
estimates with a similar multispecies treatment in Table 4.1.
The BAFP calculations in Reference 4.2 eliminated most of the assumptions that limited
Q values in pri or studies, plus they concentrated on cond itions in parameter space
where prom ising Q values could be expected. These conditions include that the
electrostatic well is deep enough (Eo
100 kV); Ion confinement time is long enough
0.01); the ion source strength is moderate (Smax < 300), wh ile the ion injection
(8
energy is slightly below the potential-well maximum. (Here 8 and S max are normalized
quantities defined in Reference 4.2) .

~

~

These results again confirm that proper formation of the electrostatic well is essential to
achieve large fusion gains, and demonstrate that the distorted Maxwellian ion
distribution-neglected in previous analyses-can play a positive role in the IEC gain .
Results also show that the square well assumption, used in previous analytical
estimates by Nevins is in fact a pessimistic one. Thus, parabolic wells result in larger
density peaks at the center, yielding Q-values 3 to 5 times larger (for Eo 150 kV) than
those obtained with square wells. Parabolic wells are also more forgiving with respect to
the source to ratio requirement. As previously noted Chacon found steady-state
distributions in which the therma lized component of the distorted ion population is
significant and can produce high gains. Such operating regimes with Q-value > 100
have been identified, as plotted in Figure 4.3. These results ignore electron
Bremsstrahlung loss however. That effect on the Q-value was addressed heuristically by
Chacon using a semi-analytic model, indicating that quite large Q-values are still
possible, provided that electron particle losses are kept small and well depths are large.

~

These results show that in addition to potential well shape, the source strength vs. up ­
scattering losses plays a crucial role in the IEC energy gain. More insight into this can
be obtained from the distribution functions for trapp-ed ions, sketched in Figure 4.3.

36

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8

Q

6

Q
2

Q

Q

rel

fdl

Figure 4.3. Plot of the Q-value (including electron losses) as a Function of (a) Smax and y with f.= 10·3 ,
(b) Smax and f a with y = 5, (c) Smu and f o with y = 100, and (d) Smax and Eo with y= 5 and f o =10"3 , These
plots have been obtained for Eo = 100 keV [except (d) ] , 9b = 0.01 , and E, = 1.04. (Nomenclature is defined in
Reference 4.2).

The source to sink issue noted several times here can be explained as follows. Two
opposite limits of this kind of solution are depicted in Figure 4.4. The realization of
either of these limits depends on the equilibrium between two competing effects,
namely up-scattering of the Maxwellian ion component confined in the well (which
increases as the Maxwellian temperature increases and tends to empty the well), and
down-scattering of the beam (which tends to fill it). The relative importance of these
effects is directly related to the strength relatives of the source and the sink. They are
characterized here by S ma x = maximum value of the ion source, and t = ion
replacement time, respectively . Thus, weak sinks and strong sources will result in a
large beam population, increasing the beam down -scattering rate and hence increasing
"effective" Maxwellian temperature, given in the dotted line profile in Figure 4.4.
Conversely, weak sources and strong sinks will result in a small beam population, thus
decreasing the beam down-scattering rate and resu lting in lower Maxwellian
temperatures, leading to the solid line profile in Figure 4.4 .

37

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I

I
I

"'

f I\
I I \
I I \
I I \
I I \
I
I
\
f I
\
I I

''

- - .. -

'

\
\

\

' ...
Emax

E

Figure 4.4. Sketch of Two Opposite Limits of the Beam-Maxwellian Equilibrium. The solid line

corresponds to a case In which the Maxwellian population Is dominant; the dashed line corresponds to
a case in which the beam contribution is dominant.

In summary, unlike the original Nevins "calculations", the subsequent Chacon, et al.
results are quite encouraging but leave open the issue of whether or not satisfactory
deep potential wells can be created. To pursue this issue further, Ivan Tzonev ,e t al.
considered well formation with emphasis on angular momentum effects (Reference
4.4) . Earlier studies had assumed that very low angular momentum (zero in the ideal
case) is necessary to achieve a potential well structure capable of trapping energetic
ions. In contrast, Tzonev et al. considered high -current ion beams as having a
significant angular-momentum spread. The results found were positive, and this is
important due to the need to create wide wells to provide a large reaction volume,
hence larger power IECs. Before discussing this work, some definitions for the potential
well structure will be reviewed.

POTENTIAL WELL STRUCTURE
The potential structures are called double potentials because two extremisms ("outer"
and "inner" wells) are observed in the plots of electrostatic potential versus IEC radius,
excluding the real cathode grid minimum. A schematic representation of a typical
calculated potential from Tzonev's work is shown in Figure 4.5. These cases are
different from Hirsch's ideal case described in Section I, where multiple potential wells
38

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with sharp peaks were observed. In Tzonev's study, spread-out potential extremisms
are observed due to the high angular momentum spread. The virtual anode is defined
as that position where the potential increases from its minimum value at the real
cathode up to about 95 percent of its maximum value. The virtual cathode is defined as
the position where the potential is 95 percent as deep as its minimum value in the
center of the IEC device. The depth of the "inner" potential minimum, more frequently
called the double well. Depth is defined as a percentage of the height of the "outer"
potential maximum. For the case shown, the double well has a depth of about 60
percent.

1BC cathode rid
I

Virtual
·albo<l

Vinual
anode

·-:: I!.

0

Radiu [m]

Figure 4.5. The Definition of the Double Well
Depth: Double Well Depth [%] = -dV /V10t x 100.
The case shown assumes 30-keV injection .

Figure 4.6. The Definition of the Parallel and
Perpendicular Velocities at the IEC Cathode Grid

The definition of angular momentum is also illustrated in Figure 4.6. In spherical
geometry, the velocity component perpendicular to the radius axis represents the
angular momentum.

TZONEV ET AL. - DEEP WELL STUDY
Tzonev et al. (Reference 4.4) used the IXL (ion accelerated code), a 1-D electrostatic
Poisson-Vlasov equation solver for use in spherical geometry. IXL wa s originally
developed by Mission Research Corporation for R.W. Bussard. The primary purpose of
the code is to determine an electrostatic potential consistent with the dynamics of the
charged particles with in that same potential, and to determine the charged particle
density distribution inside of the spherical cathode. While IXL neglects collisional
effects, it still provides an important limiting case where space charge effects dominate.
The boundary conditions for each particle population are characterized by five
parameters: injected beam current, average injection energy, energy spread associated
with the velocity component in both parallel and perpendicular directions, and the
number of recirculations through the core.
Tzonev et al. found that deep double electrostatic potential wells can occur at high ion
and electron currents (30 A-60 A); high perpendicular ion energy spread (3 keV-14
keV); low perpendicular electron energy spread (3 eV), and low radial ion energy
spread (0.1 eV-0.5 eV). An example is given in Figure 4.7.

39

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The corresponding ion density profile, shown in Figure 4.8, has a high value inside the
virtual cathode (center core plasma) and also a peak in front of the grid (real cathode).

Radius [ml

Radius [m)

Figure 4.7. The Double Well Potential Calculated With
IXL Code for d1Jperp, i=14keV, I 1= SSA, I e= S9A

Figure 4.8. Ion Density Profile for Potential
Well Shown in Figure 4.7

An important new insight obtained in
y - ml•mOAS
the study revea led that these potential
Error
profiles create ion density distribution
1.51 SSe-09
ml
9.5163c-08
80
NA
9. 1482
Ot q
functions completely different from the
70
0.99906
R
ones observed when a single well
60
electrostatic potential exists. Two ion
density peaks were commonly observed
j 50+----+-----+--+--•/'f'
- 40 _ ________,
- one in the central IEC core region, and
one near the cathode wire grid as seen
~ 30
0
in Figure 4.8. In th is manner, the single
o wr--;r---,--zr--;t:====t===:::1...,.
ion peak created by the single well
potential is split into two peaks. The
central ion peak has a much sma ller
o.i...-,.:j~.:+-t-1-+-+++-+-+-+-t~~......+-1
70
60
20
30
40
so
radius than the original peak. This
Cllhodc CWTCDI [A l
causes higher ion densities to occur in
Figure 4.9. The D-D Fusion Reaction Rate Versus
the central potential well, which is
Cathode Current for dEperp, 1 =8 keV
essential for the achievement of high
fusion rates. However, since the fusion core rad ius iin these calculations is very sma ll on the order of 0.4 cm - 0 .9 cm, the total number of neutrons emitted per second is too
low to create useful fusion power (see Figure 4.9). Still, the physics principles
Illustrated provide important insight into injection issues.

-

A reduced angular momentum spread and higher injection energies would be required
to correct the well volume problem. Sti ll , the 0 - 0 fusion rate scaling of 15 is
encouraging, and it is indeed surprising that this large angular momentum spread
achieves such distinct double well structures. As stated in Section I, the current scaling
for beam -beam reactions is strictly 12. However, as shown by Tzonev et al., nonlinear
changes in the potential well shape and ion density profile combine to cause the higher
power current scaling law. It would be anticipated, however, that this effect would
saturate at some current, tending back to the fundamental I2 relation. Along these
lines, it should be noted that prior investigators also predicted scaling laws with
40

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UNCLASSIFIED/ f FOR 8FFl@IAL ~!II! 8HLY
exponents greater than 2. The first suggestion of th ils was By R. W. Bussard based on
theoretical arguments (see Section 1). Later, PIC studies by M. Ohnish i at Kyoto
University (now at Kansai University) also showed such strong scaling (not covered
here- but see M. Ohnishi in Proceedings of 16th IEEE/NPSS, vol. 2, pp. 1468-1471). The
unanswered question is at what current level this occurs. Future simulations shou ld
address that issue and also examine low angular momentum spread and higher ion
injection energies.
In summary, this study by Tzonev et al. is very encouraging for formation of deep wells
in IEC devices designed for reactors using beam-beam dominated fusion. However,
much more work needs to be done along these lines to fu lly identify the optima l ion
injection strategy for deep wells with minimum power input. As stressed earlier, the
potential well parameters must also be combined with a consistent calculation of the
energy gain (Q) following the methods of L. Chacon et al. to establish a complete
picture of energy gain possible in a power type IEC.

MOMOTA ET AL. - STUDY OF VIRTUAL ELECTRODE STRUCTURE
In another related study, Momota and Miley (Reference 4.5) used an analytic solution
to examine the angular momentum effects. "Double-we ll " potential structure (virtual
cathode formation) was studied in a stationary spherical IEC using the nonlinear
Poison's equations and particle densities derived from kinetic theory. A novel method to
obtain a spherica lly symmetric stationary distribution function is introduced and an
integral-differential equation is simplified by applying a re levant approximated formu la
for an integral. Electron and ion beams are co llision-free, and their velocities are
roughly aligned toward th e spherical center, but with a slight divergence. Analyses
show that the angular momentum of ions and the smaller one of the electrons create a
virtual cathode, i.e., a double-well structure, of the electrostatic potential on a potentia l
hill near the center. The density limit of an !EC well was found and the conditions
relevant to form a deep potential well was presented.
These results show trends roughly similar to the numerical studies of Tzonev, et al.,
and may be useful to persons wanting to study the effects analytically.

KIM - STABILITY ANALYSIS
In addition to achieving adequate potential well trapping for net energy production, the
question of stability of the non-Maxwellian plasma in the well must be considered.
(Note that "stabi lity" is a separate question from the therma li zation of the beam-like
distribution in the IEC discussed earlier. However, they are coupled nonlinear problems
due to the fact that the distribution function used for both calculations should be
consistent). N. Krall did some earlier studies to show that the distribution in the R.W.
Bussard type Polywell IEC are stable against key instabi lities such as two -stream. These
studies however, were internal company reports and not openly published . Some
information is given, however, in Reference 1.7. More recently, H.J. Kim, in his thesis
done with G. Miley, did an in-depth study of two stream -li ke instabilities in the ion­
injected type IEC (see Reference 4.6). His work is very encouraging in that he identifies
a possible "window of stability" which depends on the injected energy distribution and
angular velocity spread . This result is summarized in Figure 4.10. The analysis is briefly
described as follows.
41

UNCLASSIFIED/ /&OA: O&&ICIAL !!ii ODt11 X

UNCLASSIFIED/ /POI\ OPPICIAL l1!E or•t I
A particle-in-cell method with explicit
time integration has generally
0.4 ~ - - - - - - - - - - - - ,
dominated ea rlier plasma simulations.
a= 10·2
However, this method is inappropriate
0.3
I;= 10·1
in multiple time scale problems
0
lC= 0.0
primarily because the explicit time-step
I
I
algorithms become numerically unstable
I
I
when they cannot resolve the fastest
I
I
time scale supported by the model.
I
--0
Indeed, the fastest time scale may be
o- orders of magn itude faster than the
0.0
dynamical time scale of interest. The
-0.1 ..............................__.__.......,....._2.........,.......,........1..................,.,o
simulation also has to resolve the
10-··
10"
10"
10
smallest length scale of system to avoid
finite-grid instabilities that require
~
Figure 4.10. Plot of Growth Rate of Spherically
resolution of spatial scales comparable
Converging/Diverging Ion-Beam Instability for the
to the Debye length. In order to
Variation of the Angular Velocity Spread, i.e., for 13 =
improve the conservations property of
T/ lecticl (Nomenclature defined in Reference 6.1}
implicit moment particle-in-cell
algorithm, H.J. Kim developed a fully implicit particle- in-cell scheme and implemented it
using a Jacobian-free Newton-Krylov algorithm that does not require actual formation
and storage of the Jacobian matrix to minimize the computational costs. The scheme
features the following properties: 1) fully implicit method where all quantities of both
the particle and the field equations are consistent at each time step, and 2) a good
property for energy conservations.
H. J. Kim's study verified that the algorithm correctly handles the typical electrostatic
modes. For example, the results agree with the linear dispersion relations for simple
limits of two cold electron counter-streaming instabilities, electron Landau damping,
and ion acoustic waves. The simulation experience presented here demonstrates the
energy conservation property of the systems and the efficacy of nonlinear solver
combined with an efficient pre-condition ing wh ich is derived from the nonlinear Poisson
equation and particle description relations. For ion acoustic waves, the maximum
variation in the total energy is much less than 0.1 percent, indicating that a fully
implicit technique performs well in that particular simulation. The number of linear and
nonlinear iterations is significantly reduced when the preconditioner is applied. In fact,
for the case of linear iterations, the iteration number of a preconditioned linear system
is 10 times smaller than that of a linear solver without preconditioner. In addition, grid
convergence test shows that the scaling of CPU time is virtually linear according to the
grid number. The time convergence test suggests that employing the largest time step
compatible with accuracy in a given calculation is the most efficient route for obtaining
the solution because the CPU time decreases as the t ime step increases.
H. J. Kim performed a normal mode analysis of the ion-ion counter-streaming instability
in a spherical inertial electrostatic confinement in order to gain insights into the ion­
injected inertial electrostatic confinement equilib rium configuration. To do this, it is
assumed that the electrostatic confinement equilibrium ion beams are proportional to
1/r2 , effectively neutralizing the background electron density. It is evident from the
analysis of cold ion beams that two -stream instability in finite spherical systems may be
excited for small beam velocities compared to those of homogeneous and infinite
42

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UNCLASSIFIED/ /POI\ OPPl@IAL li!I: 8,.Llf
plasma. When an ion beam is hot, the waves excited by the instability are ion acoustic
type waves. Then one observes that a certain temperature ratio is required before the
wave will go unstable.
He also developed a two-dimensional perturbative (~f) particle-in-cell algorithm to
simulate the electrostatic interaction of electrons and ions in spherical inertial
electrostatic confinement. of scheme is applied, because the noise level in of algorithm
is significantly reduced, compared with conventional particle-in-cell simulation. Since a
particle-in-cell simulation in two or three-dimensions is very computationally and
memory intensive, it is necessary to seek computational methods and options which
lead to solutions in a reasonable amount of time. A distributed computing approach is
implemented by using parallel libraries. Applying a uniform grid spacing in spherical
coordinates may lead to a stability problem due to singularity at the spherical center. In
order to avoid the singular ity issue, a two -dimensional rectangular domain and an
immersed boundary method are applied so that a spherical domain inside the cathode
gird of spherical electrostatic confinement is simulated by r-z coordinates. It is evident
from the result that the growth rate of instability, a ,decreasing function of the
longitudinal energy spread of ion distribution function, becomes reduced because the
large energy spread induces strong Landau damping by parallel kinetic effects. Besides
the effects of longitudinal Landau damping by the barn ions, a more monoenergetic
beam is able to stabilize the instability because a growth rate of instability can be
reduced for a higher beam density when the beam speed is fixed and located on the left
hill in the diagram of growth rate versus beam speed. The growth rate does not change
dramatically as a mode of angular perturbation increases.
The results summarized in Figure 4.10 indicated that the two-stream instability is
stabilized if the angular momentum spread of the beam ions is small enough, due to
enhanced ion densification at the center of the device. This is very encouraging for
future IEC development. However, an experimental study should be performed to verify
this result.

RIDER - ENERGY BALANCE STUDY
Todd Rider in Reference 4. 7 reports a quite different type of energy balance analysis.
While the prior papers used various methods to concentrate on up-down scattering in
various potential well configurations, Rider considered a "block diagram" type energy
flow balance to determine the net gain from a power unit. In such an analysis t he
reaction and scattering rates, plus the electron-ion equilibration rates, enter as (<av>)
averaged over the distribution functions for ions and electrons. Thus, the distribution
functions were not calculated explicitly in Rider's analysis. In addition to the base
function, any spatial variations in the distribution, such as pointed out by Tzonev et al.,
must be envisioned via the assumed distribution function. In traditional magnetic
confinement analyses a Maxwellian distribution function has generally been assumed.
In prior Tokamak studies, this technique has been widely used with reasonable
accuracy since such systems are indeed near thermal equilibrium. In sharp contrast, as
stressed repeatedly here, the IEC is not.
This uncertainly in <av> makes an analysis of the type attempted by Rider very
challenging, and leaves it open for possible criticism. In fact, it appears that Rider used
Maxwellian key averages and critics generally cite this as the cause for his pessimistic

43

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results. Rider was particularly interested in the claim that due to its beam-like non­
Maxwellian plasma, the IEC can burn "advanced fusion fuels" such as D- 3 He and p- 11 B
easier than traditiona l Maxwellian type plasma devices {Tokomaks, etc.). Thus, he
considered use of D-T, D-D, D- 3 He, 3 He- 3 He, p- 11 B and p- 6 Li fuels. Due to their high Z
components, all of these fuels must battle large energy losses via Bremsstrahlung.
These losses were evaluated using the traditional form ula, but his evaluation has a built
in bias since the loses depend heavily on the electron ion temperature ratio which in
turn depends strongly on the <av> values assumed as already discussed. Deviation
from an equilibrium electron energy distribution also strongly affects radiation emission.
Using the Maxwellian average values, he found that Bremsstrahlung losses would be
proh ibitively large for 3 He- 3 He, p- 11 B, and p- 6 Li reactors and will be a considerable
fraction of the fusion power for D- 3 He and D-D reactors limiting use to D-T. As a
corollary, he concludes in contradiction with earlier claims that it does not appear
possible for the dense central region of a reactor-grade IEC device to maintain a
significantly non-Maxwellian ion distribution or keep a low electron to ion temperature
ratio. The problem, however, is that the assumed rate constants would naturally force
this conclusion. Further, these rate constants lead to Rider's build-in result forcing the
ions to form a Maxwellian distribution with a mean energy close to the energy of the
potential. Consequently, in his analysis, ions in the energetic tail of the distribution are
lost at rates faster the fusion rate, giving low Q values.
Rider considered the Polywell type IEC and even with exceedingly optimistic
assumptions about the potential well, he found the electron losses are intolerable for all
fuels "except perhaps DT". Based on these results, Rider concludes that for the IEC
system to be used as a fusion reactor it will be necessary to find methods to
"circumvent these problems, especially the excessive Bremsstrahlung losses". Certainly
reducing radiation losses should be an ongoing study, but his pessimism appears to be
overdone.
The problem with Rider's analysis is his not using reaction and scattering rates
averaged over the non -Maxwellian distribution characteristic of an IEC reactor. This
includes both the beam -like ion distribution and the large ion -electron temperature
ratio. This very basic energy analysis should be redone with revised reaction rate data,
but to date have not been reported. Despite questions about Rider's analysis and
pessimistic conclusion, his recommendations of issues to study and overcome remain
quite valid.

NEUTRON SOURCE SIMULATIONS
Several simulation studies have focused on neutron source type IECs. In this case, as
opposed to future power reactors, the background gas is of sufficient pressure,
resulting in beam-background scaling of the fusion rate (i.e., theoretically this gives a
current x pressure scaling). Charge exchange also becomes a significant factor in device
performance. In Reference 4.8, Miley et al. used an analytical model of charge-­
exchange collisions in the IEC plasma to include ion time-of-flight and fusion neutron
generation rates. Results from the model simulating 10 mA of D+ ion current in a 30-cm
diameter IEC device at SO kV matched the experimental results of 106 fusion neutrons
per second. The model was also used to find the effects of grid diameter on neutron
yield and show that the yield scales as grid diameter is raised to the power -0.41. This
factor is very close to the experimental scaling observed from the UIUC neutron source
44

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UNCLASSIFIED/ /fOR Offl@IAL WSE 8,.Llf
gridded IECs. Further, the model provides more insight into operation in the STAR
mode. This effect, described earlier in Section 1, is summarized in Figure 4.11.
For higher pressure operation, charge exchange severally limits the number of passes
possible through the grid despite the very high effective transparency achieved by the
STAR mode. This is emphasized by results for the calculations in Reference 4 .7 shown
in Figure 4.12. (Note that related calculations by J. Khachan discussed earlier in Section
II show similar results, but emphasize the role of molecular ions at lower operating
voltages).
Equ1pobtnbaI

Ion En•'VY Ol,tnbutton11at pH

Surf~

2: cm grid rad lua, 50 kV, 4. mTorr
2.0E+07 . - - - - - - - - - , ,
11

UiE+o7

I 1.oe+o1 t - - ­
•lllffl!IJl I n,i

:

5.0800

o.c,e..oo
0

20000

4'0000

80000

ton Energy (eV)

Figure 4.11. Diagram Showing Equipotential
Surfaces of the IEC Cathode Grid and Their
Focusing Effect on a Beam of Ions in the Star
Mode Discharge at High Voltages ( > 50 keV)

Figure 4.12. Results for Calculations for Ion Energy
Distributions pt Pass

These computational resu Its are for the UIUC IEC "A-device" using a diameter grid with
conditions of 50 kV, 10 mA, and 4-cm, and background gas pressure of 4 .6 mTorr. At
this pressure, charge-exchange (CX) collisions occur qu ite frequently for D+ ions. In
their first pass through the IEC, about half of these ions CX within the cathode regio n
and are lost. After only four passes, most of the remaining ions have lost a large
amount of their original potential energy and the fusion rate from subsequent passes
becomes negligible. D 2+ ions have a smaller CX cross section and it takes about 20
passes for most of the D2+ ions to lose their energy and be lost to the grid . (D 2+ ions
and also D3+ ions are natura lly produced at diminishing quantities in ionization reactions
along with o +. As pointed out by Kha ch an his work noted D 2+ becomes more significant
in lower voltages) .
In summary, the design of an optimal IEC neutron source is seen to be quite different
from a power-producing IEC. In the source design, the grid parameters, grid/vessel
diameter ratio, chamber diameter, surface conditions, background pressure, current,
and voltage all become important parameters. In power-producing devices the ion
injection parameters-- including ion current, ion energy relative to height of the well
potential, the ion angular momentum, and the ion to electron temperature ratio, along
with the chamber diameter ---determine performance. The calculations and simulations
cited here provide much insight into these issues and also provide insight into
theoretical and computational tools for IEC study.

45

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REFERENCES
4.1
W.M. Nevins, Can inertial electrostatic confinement work beyond the ion-ion
collisional time scale?" Phys. Plasmas, Vol. 2, No. IO, October (1995) pp. 3804-3819.
4.2
L. Chacon, G. H. Miley, D. C. Barnes, and D. A. Knoll, "Energy gain calculations
in Penning fusion systems using a bounce-averaged Fokker- Planck modelz" Phys. of
Plasmas, vol. 7, no. 11, (2000) p. 4547.
4.3
T .N. Tiouririne and D. C. Barnes, Optimization of SCIF Fusion Systems", Bull.
Am. Phys. Soc., vol. 40 (1995) pp. 1665.
4.4
I.V. Tzonev, J. M. DeMora, G.H. Miley, "Effect of Large Ion Angular Momentum
Spread and High Current on Inertial Electrostatic Confinement Potential Structures",
Proc. 16th IEEENPSS Symp. On Fusion Engr. (Miley ,and Elliott, eds.) IEEE paper
95CH35852, 1476-1481 (1996).
4.5
G.H. Miley and H. Momota, "Virtual Cathode in a Stationary Spherical I nertial
Electrostatic Confinement", Fusion Science and Technology, Vol. 40, July (2001) .
4.6
H.J. Kim, "Instability Studies on a Spherical Inertial Electrostatic Confinement",
Dissertation, Submitted in partial fulfil lment for the requirements of degree of Doctor of
Philosophy, NPRE Department, University of Illinois at Urbana-Champaign, Illinois
(2006).
4.7
T.H. Rider, "A general critique of inertial-electrostatic confinement fusion
systems", Phys. Plasmas, Vol. 2, No. 6, June (1995) p. 1853.
4.8
G.H. Miley, John M. DeMora, Brian E. Jurczyk, Martin Nieto, "Computational
Studies of Collisional Processes in Inertial Electrostatic Glow Discharge Fusion Devices,"
18 th Symposium on Fusion Engineering, ( 1999) p 23.

Section V. Potential Applications
The ultimate application for IECs is for electrica l power production . This is discussed
further in Section VI. Section V concentrates on various near-term "spin off"
applications of neutron/proton/x-ray sources and also non-electrical power applications
such as space propulsion .

NEUTRON/ PROTON/XRAY SOURCES
As seen from the discussion to this point, the main application of the IEC to date has
been as a small portable neutron source for NAA. In addition, since both D-D and D- 3 He
reactions can be used for proton production, IECs have also been pursed for medial
isotope and PET scan isotope production. However, due to the need for high source
strengths to fully compete in this arena, that use is still undergoing research. Another
novel application noted earlier is the use of the IEC to simulate implantation of D+ and
He+ in candidate fusion re.actor first wall materials. Yet another novel use involves
running the IEC with reverse polarity such that the trapped electrons produce soft x46

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rays which can be extracted through a special port (requires a thin low-Z window for
transmission). This can be viewed as a small scale soft source for individual laboratory
studi es such as done at various large scale national laboratories like ANL, BNL, and LBL
(synchrotron "light sources"). However, this use has not progressed much past the
initial laboratory stud ies.
The cylindrical geometry offers a unique configuration for security inspection
applications. To illustrate this we next consider a design proposed for such a system.

INTEGRATED X-RAY /NEUTRON SOURCES
This integrated inspection system was proposed by G. Miley, et al., in Reference 5.1. It
incorporates combined 2.5 - and 14-MeV IEC neutron sources and an IEC x-ray source.
Such neutron sources have already been described, but some comments about the x­
ray version are worthwhile . The cylindrical IEC, like t he spherical one, can be converted
to an attractive tunable x- ray source with minimum alteration of the apparatus. X-ray
operation involves reversing electrode polarities and adding electron emitters along the
vessel wall. Hydrogen gas is substituted for deuterium since in this configuration the
main function of gas ions is to provide electron Bremsstrahlung emission. Intense
em ission is concentrated in a small volume surrounding the central axis due to the high
electron density formed there. The resulting x-ray energy spectrum is peaked at an
energy equ ivalent to about two-th irds of the applied voltage. In the inspection station,
a voltage around 120 kV is employed to obtain ~80 - kV x-rays with some distribution of
energies above and below this mean value. X-ray production is spread along the axis
over a long line- like region in the IEC provid ing broad area coverage for imag ing,
consistent with the broad coverage of the companion IEC neutron sources.
Consequently, the basic concept of the integrated inspection unit is to comb ine IEC
neutron and x-ray sources plus appropriate detectors into a single "package" . This
combination expands the range of elements that can be detected during a single scan
and incorporates x-ray imaging with the 3-D detector array for NAA to obtain improved
identification of the shape and location of suspected objects in a container. In cases
where the object is too large to allow good x-ray penetration, information is still
obtained from the 3-D NAA detector array.

Nuclear and Chemical Explosive Detection Techniques
Most current inspection systems concentrate on x-ray imaging. This interrogation relies
on the photoelectric effect or Compton Scattering Imaging (CSI). It provides good
localization and some geometric information for higher Z materials. The NAA technique,
including both thermal neutron analysis (TNA) and fast neutron analysis (FNA), are
powerful methods for detecting certain types of explosives. Basic elements such as
oxygen, carbon, and chlorine present in the explosives can be identified through the (n,
n'y) reaction initiated by fast neutrons. The gamma rays produced in the reaction are
characteristic of elements being interrogated. Another basic element in many
explosives, nitrogen, is usually measured through (n, y) reactions initiated by thermal
neutrons. Either steady-state or pulsed sources can be employed for TNA. However,
increased sensitivity can be obtained with pulsed FNA (PFNA). This method uses a
pulsed high-energy neutron source and the time-of-flight (TOF) diagnostics to reduce
the "noise" effect resulting from scattered neutrons and x-rays.

47

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The three detection techn iques shown in Table 5.1 have previously been used
independently in inspection units. The present integrated unit makes use of all three
simultaneously. Thus, the strengths of each device combine to provide consistent
overall coverage of elements/objects.

Table 5.1. Three Kinds of Techniques Previously Used in Explosives Detection
System
Here,, TNA and FNA represent thermal and fast neutron activation respectiv,elv
Element Identification
Nitrogen
Carbon
Oxygen
Hydrogen
Chlorine
C/0 Ratio
Background Noise

TNA

FNA

X-ray
Eleme nt Density

High
Very Low
NA
Very High
Very High
NA
Hi gh

Mediu m
Very High
1-li gh
High
High
High
Low
Pul sed FNA
provides depth
information

-

NA

Imaging: geometry & localization
inti rmali n

Limited

Application

Small sui tcase

uitca et
cargo

Confi guratio n

Co mplicated

Complicated

Su itca I ca rg ,
not good for
pl astics obj ects
Simple

Hi gh

High

Low

Operation & Maintenance

Cost

Hi gh resolution

The integrated IEC system employs a combination of PFNA/TNA methods and
conventional x-ray techniques to collect elemental information as wel l as a 3-D imaging
of any suspected objects. The 14. 1-MeV neutrons from the D-T reaction will be the
primary source for TOF FNA wh ile thermalized neutrons from a block of neutron
thermalizing plastic in front of the 2.45-MeV DD IEC will be used for TNA . This
combination will increase spatial resolution and significantly reduce the false allarm rate
to a low level compared to conventional methods which emp loy single sources and are
li mited to yield due to lack of broad area sources.

Overview for Weapons/Nuclear Material Simulation Articles
The following three categories of items provide a overview of articles of interest for
detection in inspection stations.
A. Explosives which are of high O and N, low C and H elementa l densities are listed
below with sizes ranging from 8 x 8 x 8 cm 3 to 20 x 20 x 20 cm 3 .
•

RDX (Cyclonite , C3HGNGO6) with p = 1.83 g/cm 3.

•

TNT(Trinitrotoluene, C7HsNJO6) with p = 1.654 g/cm 3 .

•

PETN(Penthrite, CsHsN4O1 2) with p = 1.773 g/cm 3.

•

TATB(Triaminotrinitrobenzene, CGHGNGOG) with p = 1.93 g/cm 3 •

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•

Tetryl (Nitramine, C7HsN sOs) with p = 1.93 g/cm 3.

•

Landmine - includes plastic explosives, such as Semtex, which is made of ROX and
PETN, and C-4, which is made of ROX.

B. Nuclear Materials such as natural uranium (U -238) and weapon grade with enriched
(U-235). Testing can be done with this natural uranium which can be obtained without
special restrictions.
C. Drugs like Cocaine (C17H21 NO4 ) and Heroin ( C21 H23NOs ) are of high in C and H, low
in O and low-medium Cl densities .
O. Common articles such as ethanol alcohol/wine (C 2H5Q) with p = 0.789 g/cm3,
clothes like silk (C3H11 N3Q5) with p = 0.789 g/cm3, and nylon (C5H11 NO) with p = 1.1
g/cm3, and metal utensils, including knives.
The list provides a guide for use of articles for simulation testing in the IEC inspection
station. Fortunately, the combination of sources in the integrated station provides good
coverage of this wide variety of items. Conventional single source units suffer " gaps" in
coverage.

Pulsed Power for the Inspection Station
Advanced materials and methods are used in its design to minimize weight and size.
This extends use of stationary units (e.g. at airports) to mobile platforms such as vans,
law enforcement vehicles or light military trucks . Figure 5.1 shows a block diagram of
the power control circuits .

Figure 5.1. Block Diagram of the IEC Pulsed Power System

The power is taken from the platform power system and can be either 12-48 VOC or
single phase 120 VAC. The high voltage power supply is a solid -state switch-mode
inverter followed by a voltage multiplier circuit that incrementally steps the voltage up
a series of capacitors fed by high voltage diodes. The energy storage unit is composed
of high voltage capacitor segments with a low series inductance and resistance. The
capacitor bank is charged and discharged at the pulse repetition rate. The custom
capacitor sections are engineered to withstand the mechanical stress caused by rapid
charge/discharge cycl ing. The nominal capacitor bank voltage is 120 kV at full pulse
power output. A triggered spark gap is used to switch rapidly a high-energy pulse of
current from the capacitor bank to pulse modulate the magnetron. The triggered gap is
a compound three-element spark gap design. The arc is sustained until the capacitor
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bank discharges. The pulse shaper is a tapped inductive line with each tap terminated
by a pulse-forming network (PFN). Each tap performs a filtering function for a discrete
Fourier function or band of frequencies, which results in a smoother, and more uniform
pulse rise and fall, and also provides more constant load impedance matching, as the
IEC tube has high input impedance. This aids in the forward coupling of energy into the
IEC by minimizing backward or reflected standing waves generated by the dynamic
impedance discontinuity presented by the spark gap and the cathode to anode circuit
path. The PFN, in effect, serves as a pulsed coupled transformer with a low input
impedance and a high output impedance to drive the IEC tube. Components for the PFN
are off-the-shelf and commercially available. The spark gap trigger is activated by an
IGBT (isolated fate bipolar transistor) driven by the power control circuits.
A stable multivibrator timer circuit is reset each time the spark gap is fired by a fast
phototransistor circuit that detects spark-generated photons. This is coupled by a light
wave-guide or optical fiber to provide high voltage isolation between the solid-state
control circuits and the high voltage sections. This optically isolated signal is input to
signal processing to give time of flight measurements. A digital or manual
potentiometer range selector controls the time delay by increasing or decreasing the
resistance in an RC network. When the capacitor in this network charges to a nominal
voltage threshold, the timer circuit changes its logic state and drives the gate of the
spark gap trigger IGBT on, and this initiates the spark gap arc. This RC time period
ramp is an analog of the energy storage capacitor charging ramp, which also begins
after the main arc occurs and drains the energy store. The longer the charge time, the
higher the charge in the energy storage capacitor bank and the higher the voltage is
when the next pulse is initiated. The voltage applied to the IEC tube is thus varied over
a nominal 40 percent range. The peak output voltage can therefore be varied over a
range of 50 kV to 120kV. There is a reciprocal relationship between the pulse repetition
frequency and the output pulse peak power.

Design of the Total Integrated Interrogation System
The integrated system is illustrated conceptually in Figure 5.2.

50

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D-0 IEC for T A

0-T IEC
for PFNA/FNA

- -c:CJ D

ctor Arr y

Luggag

I C for x- y

Top View

Figure 5.2. Schematic (view from top) of a broad coverage IEC inspection System for luggage
inspection

This unit employs two broad area neutron sources to provide both 14.1-MeV and 2.45MeV neutron coverage . The IEC Bremsstrahlung source provides broad area 80 kV x­
rays. An array of gamma and x-ray detectors is provided for simultaneous x-ray
analysis using a fuzzy logic artificial intelligence system to interpret the combined
signals. In the inspection station, shown in Figure 5.2, typical of airport luggage
inspection stations, objects being interrogated pass between sources and detection
array on a conveyor belt. For maximum efficiency a duplicate conveyor belt and
detection array (not shown in Figure 5.2) would be placed on the left hand side of the
IEC sources. For other cases, such as unloading container from a ship the detectors
system might be suspended along t he side of the vessel with containers as they are
lowered from the upper decks to the waiting cargo t r ain cars or trucks (see Figures 5.3
and 5.4 and related discussion). The challenge in these systems is that the unloading
time is short and the container is large and thick. The present system with its multi-lEC
broad coverage and fast fuzzy analysis technology is designed to overcome these
issues. Further, conventional accelerator type systems that are candidates for such use
are hampered by size and cost . Thus, they have not received wide use in sea ports. The
increased accuracy and reduced cost of the present system should enable wide use .
This would close a vital gap in our seaport/airport cargo security that has been widely
cited by critics of the present handling of port security .

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Detector Array and Analysis System
The final imaging quality is determined by neutron source yield integrated over the area
of coverage by the number of detectors, the detector configuration, the collimated
beam size, the acquisition time, and other scan-related parameters. Scintillators such
as Nal, Csl, and BGO as well as Plastic Nal (Tl) are candidates for gamma and x-ray
detectors. Due to their small size, they can be used in a large array along with TOF
spectrometry to provide detailed special information. Recently, UIUC staff has studied
use of an advanced thick Hgb detector which appears to be particularly advantageous
for this application. Hgb is a room temperature semiconductor material with an
excellent stopping power and a relatively large photo-fraction for detecting high energy
gamma rays. Assuming successful development, Hgb detectors would provide superior
array performance.
With the combined sources, the detector array receives a vast amount of information in
a short scan time. To handle this flow of data most efficiently, a fuzzy logic system is
employed. The development of this advanced fuzzy !logic system is patterned after a
methodology developed for the diagnosis of abnormal situations in nuclear reactor
safety analysis. The knowledge box employs a goal tree (and/or decision tree) for
representation. This system will use learning -based rules that evolve from extensive
simulation tests to provide training about the simultaneous occurrence of specific
elements and embedded object shapes. It will also learn to differentiate between
positive and negative data by creating rules from these test runs. Materials listed early
in the simulation study discussion would be employed. The advantage of this system is
twofold . The broad area scan analysis is optimized for fast through-put of objects while
the multiple neutron/x-ray identification reduces the probability of false identification.
The fuzzy logic system will pass the integrated item or sound an alert (suspicious object
in the container) automatically without human involvement. Humans will only be
involved when an alert is sounded. An alert occurs for a range of positive identification
of materials on to suspected identification . The multiple source detector array concept
is designed to minimize items that fall into the category of "in-between" positive
identification and clearance.
The overall concept described here is illustrated for an airplane luggage scan in Figure
5.3. Data from the detection system (TNA, PFNA and x-ray) is processed through fuzzy
logic system. If the item undergoing inspection passes this test, it continues on its way
to baggage claim . If an alert is sounded, the item can either be passed through the
machine for a second scan or be manually inspected, depending on the confidence level
of the alert.
Since the integrated system is modular, t he basic components can easily be assembled
in a variety of configurations for use at different facilities. An example is the extension
to inspection of container ships being unloaded in port as illustrated in Figure 5.4.

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Side View
RC- IEC S rles
( eutron/Jc-ray Generators) .·
D
------ •
Detector Array
Signal
30 lmaglng
&Alanning
Fuzzy logic Control Unit
Top View
Figure 5.3. Further Illustration of the Integrated System for Airport Luggage Inspection. If desired, a
second belt and detector array co1Uld be located above the IEC units to provide Increased capacity .
Detector Array
Figure 5.4. Further Illustration of the Integrated System for Ship Container Inspection
This example of a IEC inspection station provides some insight into the flexibility
offered by IEC devices for adaptation to a variety of NAA-type applications. We next
53
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move to high power IEC operations which are non-electrical. Deep space propulsion is
used as an example.

SPACE PROPULSION
The IEC can be used for both near-term electrically driven thrusters for satellite
operations (using the jet mode discussed earlier) and for fusion-powered deep space
propulsion. The latter is illustrated in Reference 5.3 where a D- 3 He reactor was utilized .
This design concept is briefly summarized here.

Figure S.S. Image of a Fusion II Spaceship, a 7S0-MWe IEC Fusion-Powered Manned Spacecraft With
Ion Thruster Propulsion

The IEC Spaceship "Fusion Ship II" is shown in Figures 5.5 and 5.6. (Note: Fusion Sh ip
II represents an upgrade to high power of an earlier design termed "Fusion Sh ip I").
The overall spaceship length is 300 meters and the initial mass at mission start is 500
metric tons. Crew and avionics/computers are located in the central compartment at
the forward end of the veh icle. The crew compartment uses 12-m diameter chamber
and could contain a rotating centrifuge for sleep and exercise. A steerable antenna
located on the side of this module provides communication. Twin 175-meter long
assemblies, comprised of 5 D- 3 He spherical IEC reactors and Traveling Wave Direct
Energy Converters (TWDECs) each, generate 1394 MW of 14.7- MeV proton flux and
469-MW of thermal heat which is converted to 1197 MWe of RF electric power. 242 MW
of the electrical power, then, re-circulates to run the reactors while 750 MWe is used to
drive ion thrusters wh ile the remainder of the energy is rejected as waste heat. A fusion
fuel re-circulation and separation system is operated continuously to remove the fusion
product 4 He from the D- 3 He reactants. Any unburned fuel is co ll ected via the TWDEC
energy convertors and recirculated back to the IEC reactors. This conserves the
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valuable fuels, especially t he 3 He. The ion thrusters run on Argon propellant at a
specific impulse of 35,000 sec and an efficiency of 90 percent. The thrust is produced
4370 Newtons, probably an initia l acceleration of .0087 m/s 2 . A typical trip time for an
out-and-back mission to Jupiter is 210 days out and 153 days to return. This is
comparable to or faster than that predicted in prior fusion studies.
B • vi w

Figure 5.6. Scale Schematic of Fusion Ship II, a 750-MWe IEC Fusion Spacecraft

The initial spaceship mass at mission is 500 metric tons start; 222 metric tons are the
Argon propellant needed for a Jupiter round trip with /:::,,.V of 220 km/s. The IEC reactor,
direct energy converters and ion thrusters contribute 178 metric tons. The remaining
100 metric tons includes 20 metric tons for the crew areas, 15 metric tons for the
electronics/computers, 20 metric tons for food and life support, 15 tons for crew
shielding and 1.2 metric tons for the antenna. Inclusion of a contingency of 30 percent
of the dry mass adds 60 metric tons and provides a 30-day "safety factor" to the round
trip flight time.
Human factors have not been fully evaluated in this design, but are thought to be
acceptable with the short mission time achieved. Fusion Ship II would be one of the
largest propelled vehicles ever built, although its mass would be ¼ that of the Space
Shuttle at liftoff.
Table 5.2 compares IEC Fusion Ship I and II designs with a concept based on an
"advanced" spherical Tokamak reactor "scaled up" from t he spherical Tokamak
experiment at Princeton's Plasma Physics Laboratory as reported in an NASA-GLENN
Laboratory study in 2001. The Tokama k has a shorter trip time by employing a power
level that is six times the IEC units. Further, it is designed for D-T use (D- 3 He is difficult
to burn in such Tokomaks), but tritium handling, radiation damage, and radioactivity
Issues are not addressed. Thus, if the IEC and Tokamak were compared on the same
operational basis (i.e., same fuels and power levels), the IEC would clearly show a
distinct advantage. Note that this is even true with the Tokamak using a very
"advanced" conceptual design well beyond reach of ITER (originally International
Thermonuclear Experimental Reactor) technology.

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Table 5.2. Comparison of IEC Design and Magnetic Fusion Design

0 emll Mass Metric T

0 emJI Length (m

Pu ionShipI
500

Jupiter ne way trip time (day'

pherical Tokamak

1690

240

10
750
9

6- 12

21

217

7895

Krypt nion
I 000

Arg n ion
,000
210

H1 - magnetic nozzle

4

Rea tor power (MW)
Thrust y t m
pecific impul

hi pII
500

00

174
JO

umber of crew
Thro I P wer (MW)
Rea tor gain

r'U ion

400

48 0
7

5,435
118

The design of Space Ship II uses "coupled" IEC reactors that use magnetic guide
channels. This concept, termed the Magnetically-Channeled Spherical IEC Array (MCSA)
concept is briefly discussed next (Reference 5.3).

MAGNETICALLY- CHANNELED SPHERICAL IEC ARRAY {MCSA)
CONCEPT
The Magnetically-Channeled Spherical IEC Array (MCSA) concept for a hybrid magnetic
assisted spherical IEC configuration maintains the basic ion-injected IEC reactor
configuration but adds magnetic channels for coupling exhaust plasma and reaction
products. The MCSA is illustrated conceptually in Figure 5.7. The SIEC is confined in a
hexapole field configuration is qu ite different from the hexapole field used in R.W.
Bussard's Polywell. It is in turn located in a field channel (Bz field) created by a column
of Helmholtz coils. The field strengths of these coil sets is adjusted such that the fields
cancel in the center of the IEC, giving a larger field null region compared to that in a
cusp confined field . The MCSA configuration reta ins t he advantage of stability due to
good field curvature obtai ned in a cusp. In addition, it also effectively closes the "belt"
loss cone. As shown later, leakage in that direction is led around the hexapole coils and
back into the confinement region, termed here as "recirculation." Losses include
scattering into the spinda l loss cone along the z-axis and stochastic scattering due to
the violation of adiabatic invariance in the field null region. However, with the present
channel configuration, the axial losses from one IEC configuration enter a neighboring
unit. Thus, as they pass through the field null reg ion in that unit, stochastic scattering
leads to "retrapping" of much of this flow. Experimental verification of this "retrapping"
is then a second physics Proof-of-Principle (POP) objective. In this fash ion, an array of
multiple IECs increase the overall confinement time roughly in proportion to the number
of units. In operation, electrons would be magnetically confined as described, providing
electrostatic confinement of ions injected into the Spherical IEC region. An additional
benefit of this configuration is that in a reactor embodiment, both leaking fuel plasma
and energetic charged fusion products (e.g. the 14- MeV proton from D- 3 He), can be
collimated and aimed into a direct energy converter such as a TWDEC (Reference 5.4) .
This results in a high overall energy conversion efficiency. Alternately, for space
propulsion, the proton beam, augmented by injection of heavy atoms to increase the
flow mass, can be directly exhausted for thrust.

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Confined
¥ ' Plasma
-·•·
Sol nod
Cotl
Floatm
emholtz
Field Cotl.s
Figure 5.7. Illustration of a Three-Unit MCSA Device
Next we consider in more detail the new physics concepts, recirculation and retrapping,
that provide the basis for magnet channe l coupling.
Recirculation of Radial Belt Cone Losses
The field configuration and a hypothetical particle trajectory through a belt cusp are
shown in Figure 5.8. Because the magnetic field lines that exit the cusp -like field
reconnect to the confinement region within the coils, plasma particles that exit through
the belt cusp loss cone will recirculate back into the confinement region. This
recirculation significantly increases the confinement time verses that of a simple cusp
devise, leading to a favorable concept. The linear scaling of an MCSA fusion device with
plasma radius, as well as successful operations of multiple units in the array, is in large
part controlled by this recirculation effect.
Recirculation
.icle
. ._ l
I
II
\ , /: : I~- .._Ir .
I
\
~
/..r'"
.•
,·
_,,,.-""
Figure 5.8. Diagram of Belt-Cu,sp Fields and Particle Recirculation
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Retrapping of Axial - Loss Particles
Axial plasma particle loss escaping out through the end cone (spindle cone) of the first
MCSA unit should be retrapped in the second unit because of "KAM effective
scattering". This KAM scattering occurs as particles move into the null-field region
within each SIEC unit. Figure 5.9 illustrates the axial magnetic field component along
the centerline of a two-unit MCSA . Particles escaping from the low fie ld region of one
unit pass into the neighboring unit through the high magnetic field region between the
two-units. Upon entering the IEC region, the direction of the particle is effectively
randomized (i.e. Kam Scatter) when it loses adiabatic invariance in the field null region .
Although particles with high velocities parallel to the magnetic field will not experience
as much effective scattering, a majority of the particles entering the null field region
should experience scattering and retrapping.

Lmi,r-Fi ld

High-Field
Magnetic hannd

120 - . - - - - - - . . . . -...,...., - ~ - - . - - - - r - - - r --r¼'..Ggt.0 0,..........-------,
100
£

O>

C

Q)

C

O

·-:c,

Q)
.=

80

60

~~en 40

c a,-o

~~~ 20
0
-20
Axial Position (m)
Figure 5.9. Axial Magnetic Field Strength Along Two-Unit MCSA Centerline

This KAM scattering process is illustrated conceptually in Figure 5. 10. Before entering
the null-field region, particles have a gyroradlus that depends on their perpendicular
velocity. If v is large compared to vu, the effective scattering angle wi ll be large,
resulting in retrapping in the case of the MCSA. In the high-field region, the gyro-radius
of the particle is small. When the low-field region is reached, the particle moves in a
straight-line path along the vector direction at the edge of the null region. Since the
phase of the gyro motion of the particles is random at this point, the vector direction
they assume in crossing the null is random. The result then can be viewed, as a
random, co ll isionless, scattering process. Particles that stream along the axis and enter
a neighboring IEC scatter in this way and become confined in the neighboring IEC
(termed axial - loss "re-trapping"). This re-trapping greatly increases the confinement
time of a MCSA fusion device.

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In summary, if the physics principles
are demonstrated successfully, the
MCSA fusion power plant will offer
numerous advantages for space
propulsion as well as for terrestrial
power sources. The non-Maxwellian
plasma involved is well suited for
operation with D- 3 He fuel that provides
much of the energy output in the form
of very energetic 14-MeV protons.
Proton energy can be employed for
either direct conversion to electricity or
directed thrust . As demonstrated in the
Fusion Ship II design study, the MCSA
Figure 5.10. Diagram of Direction Randomization
(KAM effect) Due to the Magnetic Field Null Region
power plant is especially well suited for
in the IEC
manned deep space missions where the
high power-to-weight of the MCSA enables fast trip times. The concept introduces
several important new physics. The basic concept of the MCSA system is use of
recirculation from escaping particles from one unit into the neighboring one where
retrapping occurs. These physical principles have not yet been demonstrated
experimentally, but formulated initial tests could be done with a modest size
experiment and stepped up in size to eventually demonstrate a fu ll -scale prototype
power unit.

CONCLUDING REMARKS
This section has concentrated on various near-term "spin off" applications of
neutron/proton/x-ray sources and also non-electrical power appli cations such as space
propulsion. The homeland security inspection station described here show how versatile
the IEC concept is, allowing multi-device broad area inspection. Space propulsion and
related space power seems quite far term due to the large size powers demanded for
such applications. However, the possibility of using magnetic coupling of smaller unit
provides a modular approach that has many advantages, plus expediting development
by allowing concentration on the smaller modules. Iin addition, use of near-tern
electrically driven IEC jet plasma devices allows a "product" along the way to the
eventual development of the high power fusion devices.

REFERENCES
5.1

G. H. Miley, L. Wu, H. J. Kim, Nuclear Techniques in National Security Studies on
Contraband Detections !EC-based neutron generator for security inspection system",
Journal of Radioanalytical and Nuclear Chemistry, Vol. 263, No. 1 (2005) pp.159164.

5.2

R. Burton, H. Momota, N. Richardson, Y. Shaban, and G. H. Miley, "Fusion Ship

II- A Fast Manned Interplanetary Space Vehicle Using Inertial Electrostatic Fusion",
Space Technology and Applications International Forum - STAIF-2003, Albuquerque,
NM, 654, Feb 2- 5 (2003) (M.S. EI-Genk, ed.), American Institute of Physics Conf.
Proceedings, pp. 553-562.

59

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5.3
G. H. Miley, R. Stubbers, J. Webber, H. Momota, "Magnetically-Channeled SIEC
Array (MCSA) Fusion Device fo r Interplanetary Missions", Space Technology and
Applications International Forum-STAIF 2004 (M .S. El Genk, Ed.) American Institute
of Physics Conf. Proceedings (2004).
5.4

H. Momota, G. H. Miley, and J. Nadler, "Direct Energy Conversion for IEC
Propulsions", Report to National Institute for Fusion Science of Japan, Report NISF641, ISSN 0915-633 X, August (2000).

Y. Gu, M. Williams, R. Stubbers, G. Miley, "Pulsed Operation of Spherical Inertial­
Electrostatic Confinement Device," Proceeding of 12th Topical Meeting on the
Technology of Fusion Energy, ANS, Reno, NV, 16-20 June (1996), pp. 1342-1346.

5.5

Section VI. Possible Next Step Breakeven Experiment
The prior sections have presented much information about the existing data base and
theory for IEC operation. The potential for use in applications such as a neutron source
and related radiation sources (proton and x-ray) are well established. However the
ultimate goal is to develop a power-producing IEC. Better yet to do this taking
advantage of the unique ability of the IEC to use non -Maxwellian plasma to burn
advanced fuels to minimize radioactive and radiation emission involvement. However
the best current device resu lts are 5 or 6 orders of magnitude down in energy gain Q
(energy out/ in) from breakeven. Thus it may appear that such a hope is many years
off. Fortunately, the IEC can be scaled up in energy gain while keeping a small size
since the losses are in velocity space (i.e. via ion upscattering out of the potential well
trap). This is in sharp contrast to Tokomaks where loses occur via diffusion across the
outer surface, so Increased confinement times have been achieved by going to the
massively large ITER type devices. The problems and costs for construction of ITER
have thrown its development in to the distant future, making this approach ineffective
for addressing the present energy crisis (or as a LLNL associate director recently
bemoaned, "Fusion is irrelevant- no politicians even mention it In the energy scenario").
To provide the reader with some insight into the IEC "vision" for power, we next
present a conceptual proposal for a near term IEC breakeven experiment to prove the
physics of operation with aneutronic p- 11 B fuel. If such a program can be initiated
aggressively, the IEC cou ld have a major impact on the energy crises .

DEMONSTRATION OF NET ENERGY GAIN USING IEC ANEUTRONIC
FUSION
The IEC is one of the few approaches to fusion that has the potential of burning
aneutronic fuels such as D- 3 He and p- 11 B in a reasonable scale device. This fue l results
in charged-particle reaction products wh ich allow efficient use of direct energy
conversion technology with no direct greenhouse emissions and minimal radioactivity or
radioactive wastes. Such a power source has all of t he features sought for future power
plants needed worldwide to turn the tide of the growing energy crisis. The experiment
proposed here would provide verifiable and reproducible proof of break-even conditions
necessary to burn p- 11 B as a practical aneutronic fuel in an IEC fusion power-generating
device.

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The proposal is to develop a revolutionary small IEC fus ion power unit that could be
commercialized in time to impact the energy crisis we now face. This device would have
the aggressive goal of burning relatively inexpensive aneutronic p- 11 B fuel, avoiding
issues of tritium breeding and radioactivity that the D-T burning ITER type devices face.
This breakthrough technology is the resu lt of new understanding of ways to create a
deep potential electrostatic 'well' for improved confinement in an ion injected IEC. This
will be done with specially designed ion guns to inject ions into the IEC with strong
focus and controlled angular momentum. This concept builds on a combination of prior
sma ll scale experiments with gun injected IECs and simulation of their scale-up to
power production using particle-in-cell (PIC) codes and particle tracking analysis
described in earlier sections. The small size of the IEC is a key characteristic. If rapid
development is to be achieved, the ability to employ small size experiments is essential.
Fortunately, confinement scaling in the IEC is in velocity space (vs. physical space
which brings in reducing the surface-to-volume ratio), allowing breakeven and power
production in small-volume plasmas .
Thus in principle, energy breakeven could be demonstrated in the IEC in a very dense
plasma "core" occupying only a few cc volume with only a few l00's of watts in and
out. This extreme is not currently possible, but use of the new gun injected technology
to obtain breakeven in a dense plasma core in the IEC of l00's of cc volume and with
20-25 kW input power seems practical. This proof-of-principle device would
demonstrate the physics of energy production and prnvide the basis for going rapidly
going to practical IEC power plants. This route could lead to power reactors for
distributed power applications in the range of a MW that are only a fraction of the size
of an ITER Tokamak type plant or even current fission nuclear plants. Small units of this
type could be rapid ly deployed to allow fusion power to have a real impact on t he
growing need for energy. Fusion power would then become a vital element in our goal
of energy independence.

VISION OF A FUTURE P- 11 8 FUSION PLANT
In the ultimate power plant, the preferred fusion reaction would employ aneutronic p11 B fue l, which fuses to produce energetic alpha particles with no neutrons and minimal
radioactivity. This eliminates radioactive tritium breeding and corresponding tritium
inventory, activation and damage to reactor structural materials and the massive
shielding and radiation protection in traditional fission and D-T fusion reactor systems.
In this case, p- 11 B reactors in the central IEC core result in MeV energy alpha particles
according to the reaction: p+ 11 B 3a. Due to its inherent non-Maxwellian (beam - like)
plasma, the IEC is especially well suited for burning a fuel such as p- 11 B which requires
high energies (~150 keV) . [See Figure 6.1]. In operation, a bulk of the IEC driving
energy is given to ions so an applied voltage of~ 180 kV provides ion energ ies nea r the
peak of the p- 11 B cross section. In contrast, in Maxwellian-type plasmas typical of
magnetic confinement devices, energy is expended t o create ions over a wide
distribution of energies. Thus, Tokomaks are designed to operate at much lower ion
energies (~20-30 keV) suitable for D-T fusion. The key physics challenge then for the
IEC is to achieve good ion confinement via strong ion trapping (i.e. larg e number of
recirculations) in the potential well. This trapped plasma must meet the Lawson
criterion for energy to break-even with p- 11 B, n, ~ 10 16 cm· 3 - sec (two orders of
magnitude above the requirement for D-T fusion). Here n is the ion density and -r is the
ion confinement time. Assuming the converged core density in the potential well of
61

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~10 16 cm -3 , ion trap times of~ 1 sec are required. While very demanding, plasma
simulations show that carefully controlled injection can provide the potential well
formation (trap) required to achieve this goal.

PROPOSED BREAKEVEN EXPERIMENT
Studies of gridded configurations discussed earlier have achieved reaction rates of up to
10 12 reactions per second(~ 1 Watt of fusion power) . Although these power levels fall
we ll below the requisite "break even" condition for power production, corresponding
neutrons production makes the IEC an excellent compact source for practical NAA.
Consequently this application and then related "spin -off" type projects have continued
to advance IEC basic physics understanding to the point where a pathway to a power
reactor can now be envisioned. Present IEC experiments at UIUC are designed to
baseline Q impact of ion injection conditions combined with supplemental electron
sources to maintain the desired quasi-neutrality. One of the current experiments,
shown in Figure 6.2, consists of a 16-inch diam . spherical vacuum system with a
spherical grid held at a high potential. This system produces about 108 reactions/sec
based on neutron counting experiments. A specially designed radio frequency (RF) ion
gun is installed on the side of the chamber to study controlled ion injection and
corresponding potential well formation for ion trapping.
Spherical f(v)

10-2 ,
1

- T77"l.i:~=t~~
=1 ;:----:---:-:-:;;-~----~I""7

10-2 2
l

i

Ill

10-23

'E
'-'
G>

ii
10-24
a..
C
0

:.:.
u
«I
G>

~

10-25

10-26

10-27 ~ - ~ - - - ~- - - ~ - ~ - ~ - ~ - - ~ ~
1.
2. 3. 5.
10.
20. 30. 50. 100. 200300
IEC well depth ( kV )
Figure 6.1. p- 11 B Fusion Cross Section Energy Requirements. The p- ll B react ion rate
approaches that of D-T at very hig h energ ies, I.e. deep potential wells.

62

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These studies did not yet include differential pumping so that number of recirculation
passes by an ion , ~, was low, roughly 2, due to major charge exchange losses. The
injected ion current, I, with one gun was only~ 50 mA. Sti ll, based on measurements
of neutrons emitted using deuterium fuel, the Q (fusion energy gain/energy in) was
order of 10-6 which is remarkable for such a small device. These results, plus supporting
computer simulation studies, show that the scale-up of th is device to 12 injector guns
plus adding strong differential pumping, could potentially achieve breakeven.
The proposed ion injected IEC device
with 12 guns is shown schematically in
Figure 6.3. The key to ach ieving
breakeven conditions in this device is to
inject ions with good focus and the
desired angular momentum. The RF ion
gun has a unique magneti,c nozzle to
achieve that . Electrons are
simulateously introduced in a measured
fashion. This eliminates the need for a
grid by formation of a deep potential
well (ion trap) . Also, differential
pumping between the guns and the
main chamber provides the high
vacuum needed to avoid charge
exchange. This configurat'ion is highly
Figure 6.2. IEC system With Radio Frequency Ion
non -Maxwellian due to the beam
Gun
dominated nature of the trapped ions.
(Although, as pointed out in the disuccion of L. Chacon's work, therma lized ions build
up and additiona l qua si- Maxwell ian dist riution in the trap) . Still, detailed analysis such
as done by Momota and Kim (discussed earlier) shows that the beam ion momentum
provided sufficient "stiffness" to the system to maintain stability. This assumes,
however, very precise control is maintained over the energy and angular momentum of
injected ions and a balanced supply of electrons is provided. An RF ion injector capable
of such operation has been demonstrated at UIUC as discussed earlier.
mapctic ~utm1 lem

.,1,mi..,,..1111,.,.

pot 1C l\ew111 lL
ptrt ol ,'::.1 cui..Lmck.ntbe:r -

coufal ~,pcrr1ti0 111tot: OOIJaw c:ybndrtcl.l

I

HIJt

b,li<ol aa,.nm

1k dJl'P.1 1'tt1u.1 l colb

j p,s lli>•

Rl'r"""t°'

Figure 6.3. Multiple Ion Gun Concept

Figure 6.4. Differentially Pumped RF-Driven Ion
Gun. Six guns shown fo r si mplicity, but t welve are
proposed for th e brea keven study .

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Figure 4 is a schematic sketch of the ion injectors shown in Figure 6.3. A key
component is the magnetic focusing lens at the gun extraction port. This allows very
efficient differential pumping between the high pressure gun chamber and the low
pressure IEC chamber and provides focus control. This experiment will involve very
high power inputs (about a MW). To avoid excessive power supply and thermal
controls, a Marx bank pulsed power input with peak powers of ~ 1 MW over 1 msec at
0.01 Hz will be used. Pulsed experiments with equivalent power inputs on one gun have
already been performed successfully. The pulse len9th is set long enough to provide
quasi equilibrium physics conditions in the trapped plasma during the "flat top" region
of the pulse". Thus the data obtained is relevant to eventful steady-state reactors
where the internal fusion power production alleviates the input power supply
requirement.
The energy gain (Q) scaling for such a device goes as ~l/a 2 where a is the radius of the
dense core spot formed in the IEC sphere, ~ is the number of ion recirculations in the
trap before the ion is lost, and I is the ion injected current. The 12 gun breakeven
design will provide an increase in ~ to ~ 1000 due to differential pumping effects, I will
increase to 6000 mA (due to multiple pulsed guns), and a will be cut down by 10 due to
improvements in focusing both and reduced collisionality. This predicts an increase in Q
(compared to prior gun experiment) of ~10 8 , giving Q=l ("breakeven") as required for
a p- 11 B plasma (as noted earlier, this breakeven Q assumes a Lawson breakeven
confinement parameter of m that exceeds the DT requirement by a factor of 100. In
other words, this could also be thought of as a Q=lOO DT equivalent breakeven!) Such
an experiment would provide a physics proof-of-principle for this ion injected IEC
concept and provide the base for extending this configuration on to a power-producing
plant.
To accomplish this result quickly on a modest budget, we need to simplify the work by
avoiding t he need to develop new injection technology for hydrogen-boron fuel plus
avoid the need to handle the fusion energy produced. Thus we propose to confirm the
achievement of breakeven conditions using a hydrogen plasma and diagnostics to show
that them and T corresponding to Q = 1 (p- 11 B) are obtained. (An alternate approach
might be to use deuterium as is done in present IEC neutron source studies. However,
that would require a massive shielding and other access restrictions for the neuron flux
levels) . Modern plasma diagnostics can make quite precise measurement of the plasma
conditions needed for the confirmation, so the hydrogen equivalent approach is
recommended.

CONCLUDING REMARKS
Once achieved in hydrogen, these conditions could be fairly quickly confirmed with p11 B fuel in later experiments once the needed fuel handling system is added. Thus the
proposed hydrogen simulation of p- 11 B breakthrough conditions would be a landmark
achievement, leading the way to rapid deployment of the technology needed to build
small fusion power plants. The technology development needed to proceed largely
involves the design and engineering of subsystems for the balance-of-plant (BOP).
Many of these can employ conventional equ ipment, but several require new
developments. These include the hydrogen-boron fuel injection system, the direct
energy conversion system to convert the charged particle product energy to electricity,
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and the exhaust plasma collection/recovery system. Also the chamber wall must
incorporate advanced cooling methods to handle the large surface heat loads caused by
near surface absorption of the bremmstrahlung emission. There are no "show stoppers"
however, so the road map to !EC fusion power seems clear. The main challenge is to
find funding sources with the "will" to proceed.

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