AAWSA DIRD Inertial Electrostatic Confinement Fusion March 10 2010
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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
this document should be addressed to !AAP Person 1 I
AAWSA Prog ram
Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000 , Washington,
DC 20340- 5100 .
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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 ........................ . ..................................................................... 11 • • • • • • • • • 32
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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
System for Luggage Inspection ··············································••11••·······
Figure 5.3. Further Illustration of the Integrated System for Airport Luggage
51
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 p-
11 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
.....-
iR .._o •u:s
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
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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 Figure 1.4. Photo of Star Mode Seen Through a Reaction Vessel
Gridded Devices Identified by Port Window
Miley
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
10
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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
stainless steel flange
l
ceramic (insulator) upper plasma stream: floating
negative potential
Q~ n beam~t,,J;,l;~==;:;===l:~l=t'A'Ft,VtJil/1t:tl't~l=l:~ .l4.,._t:t,:==;=f=i=r"iF=;;:=l
D2 gas feed
positive wall, j l
magnetic differential coils
part of vacuum chamber ~ helical antenna ! glass tube
RF generator
lower plasma stream: floating
Figure 1.5. Schematic of the UIUC RF Gun Injector for IEC Experiments
11
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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 Figure 1.7. Photo of Center Spot Formation. The
the UIUC Laboratory 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. 2502-
2511.
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- ·--,
' :
~
HIGH VOL.TAG[
DOME 0
CONU,INING u
w
109
[g AUXIL A,tY
POWER
SUPPI. IES
ll)
....
~
0
Q::
t-
~
tHGH VOLTAGE
INSUL. ATOA .
co.I
I-
:,
108
0.
6
107
I-
:J
.z
106
..
AP PUEO POT Tl~L - V
Figure 2.1. The "Historic" Early IEC Ion Injection Figure 2 .2. Neutron Rates Measured With the IEC of
Experiment of R. Hirsch Working With Philo Figure 2,1 Exceeded 10 9 OT n/s at 150 kV. For
Farnsworth 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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UNCLASSIFIED/ /fOR Offl@IAL ~SE 8,.Llf
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.
15
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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.
in these configurations. (Indeed various The "vane'' type grid shown Is only one of a number of
multi-grid approaches have been studied large opening grids designed and used for STAR mode
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,
optics for reflection of ions, hence Daimler-Chrysler and Kyoto University. It Is rugged,
recirculation. See for example, Reference shows little sputtering and has proven very efficient for
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
16
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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 R O of compact IEC
• Diagnostics~ • C /pulse IEC;
Kyo o-U . T T, 'f\J uu-U. Kyo'o-U, K
• Tomography; • CW/Pu se power
I -bay E rgy 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 -~-- ~ ~
I
,..., ·i-~~ ~
neutron ta111• - - -~...
source • ., \ - I
y~ay ~- ~
detector Alo • • T explosive
I 3 , 3 I~ I
Ato, 1 r io of iv fix d
y ray en i ion
N(n, y) • • • 10.83 MeV y ray n i 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
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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)
~
Reflector /
I
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 p-
11 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-te
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