AAWSAP DIRD, Inertial Electrostatic Confinement Fusion, March 2010

UNCLASSIFIED//POI\ OPPl@IAL ~SE 8Ptl¥ Defense Intelligence Reference Document Acquisition Threat Support 10 March 2010 ICOD: 1 December 2009 DIA-08- 1003-006 Inertial Electrostatic Confinement Fusion UNCLASSIFIED//FOA OFFIGial.k Wli& OP•k¥ UNCLASSIFIED/}F8R 8FFl&IAL W&& 8flki\f 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 Program Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington, DC 20340-5100. UNCLASSIFIED//liQA QFfilGIAk W&E 8HL?f ii UNCLASSIFIED/,'FOR OFFIElIAk WSE OPtklf 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-11B 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 iii UNCLASSIFIED/ ,CFOA OFFIQIAk Wliii 01\lk¥ UNCLASSIFIED/ }FOA OFFICl.'1.k W&li Ol\lk¥ 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-118 Fusion Plant .......................................................,......... 61 Proposed Breakeven Experiment...................................................................... 62 Concluding Remarks......................................................................................... 64 UNCLASSIFIED/ /FOA OFFICIAk Wlili 0PU:?f iv UNCLASSIFIED/,SFOA OFFI&IAk WSE 8Ptklf Figures Figure 1.1. An UIUC Spherical IEC .............................................................................. 1 Figure 1.2. Idealized Potential Structure Calculated by Hirsch for Monoenergetic Figure 2.2. Neutron Rates Measured With the IEC: Neutron Rates Measured With Figure 2.5. Scheme for Landmine Detection Using a Hybrid Magnetron Type Figure 2.6. 3He Ion Source for Use in 3He - 3He Fusion Rate Studies at the U Of Figure 3.3. Illustration of the IEC Jet Thruster Experiment Showing Central Figure 4.6. The Definition of the Parallel and Perpendicular Velocities at the IEC Figure 4.10. Plot of Growth Rate of Spherically Converging/Diverging Ion-Beam 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 the IEC of Figure 2.1 Exceeded 109 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 IEC Neutron Source ............................................................................ 18 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 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 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 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 1st 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••······· 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 UNCLASSIFIED/ ,CFOA OFFI&IAk WSE 8PtLY V UNCLASSIFIED//FOR 8ffl@IAL t.1!11!! eHLY 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- 11B 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 UNCLASSIFIED/ /FOR OFFl@IAL l?ISE O,.L\f vi UNCLASSIFIED/fFOA QFFIEIAL ~81: 8HLY 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- 11B (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 11B. 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- 11B using a hydrogen plasma simulation. The author has purposely tried to avoid use of ,equations 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. UNCLASSIFIED/ fFOA OFFICI.IJ.k Wii Ol\lk¥ vii UNCLASSIFIED/;CfOA OFFI&IAk WSE 8Ptklf 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 -l- Power Supply 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 highly charged negative grid. A photograph of a typical 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 through the grid openings. This is Important for long run times since 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 probability 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 UNCLASSIFIED/ / EAR OFFI CI O Li !liF QNL¥ 1 UNCLASSIFIED/ /P"Olt OP"P"lelAL tt!I!! er•tY 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 = 1014 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 ~ 1014 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 1024, 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-3He and p- 118, 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 ~ 1016, 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 acceleration 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 UNCLASSIFIED/ /FOR &FFICI0~ 1155 AN! X 2 UNCLASSIFIED/ /POI\ orr1e1At li!II!! OHL¥ 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 T1 and Te are the ion and electron "temperatures", respectively). Control of this 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- 11B (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- 11B 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 UNCLASSIFIED/ /EAR OFFICIO! !!SF ON! X 3 UNCLASSIFIED/ /FOR OFFI&IAk W&E 8Ptklf 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 National 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 similar 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-3He and p- 11B 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 small 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, hopefully 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 EMC2 on the Bussard Polywell device or the advanced gridded IEC neuron/proton source 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 the reader is encouraged to discuss that work with those individuals directly. UNCLASSIFIED/ /POI\ 0rr1e1At l?ISE 8Ptklf 4 UNCLASSIFIED//fOft Offl@IAL l!ISE 8P•tlf 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 stabilize 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 which "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. UNCLASSIFIED/ }FOA: OFFICIAL !!ii onn X 5 UNCLASSIFIED/ /FOR 8FFIOIAL ~81!! 9HLY .....­ 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 cathodes observed were originally termed "poissors" by the 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 example, 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 charge 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 UNCLASSIFIED/I f8R 8FFIEilAk lel&& 0PU,¥ 6 UNCLASSIFIED/ /FOR 8FFIEJIAL ~SE er•tv 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, avoiding 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 central 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 UNCLASSIFIED/ /P"Olt 8FFIEJIAk Wliii QrslL¥ 7 UNCLASSIFIED/ /POI\ OPPl@IAL ~SE &PU:¥ 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 addition 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 cm3; 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 openings 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." UNCLASSIFIED/ )FOA OFFIQIAk Wliii 01\lk¥ 8 UNCLASSIFIED!/POI\ OPPlelAL ~SE &PU:¥ ' , I I I, I • I \ I I I I I I, ... ___ ,.,. ; Figure 1.4. Photo of Star Mode Seen Through a Reaction Vessel Port Window Figure 1.3. Discharge Modes in Gridded Devices Identified by 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 forming the potential well. In order to maintain 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 Penning trap, the Nebel POPS device, and the Miley ion injected device. UNCLASSIFIED/ /F8R 8ffl@IAL l:t!!! or•t I 9 UNCLASSIFIED// FOA OFFl&ilAk lalli& O,.klf 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 EMC2 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 reasonable 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 gain 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 internal oscillating ion cloud may undergo a self-similar collapse in a harmonic oscillator potential formed by a uniform electron background. This then forms a dynamic IEC device, but with a quite different ion distribution factor vs. the "conventional" beam -like 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 previous work in a systematic fashion by developing a formalism that determines the required velocity distribution of UNCLASSIFIED//509 OFFICI 0l 11ili Alll¥ 10 UNCLASSIFIED//FOR 8FFI@IAL ~81!! 9HLY 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 EMC2 . 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 this design is shown in Figure 1.5. magnetic focusing lens stainless steel flange ceramic (insulator) negative potential Q ~ nbeam~t,,J;,l;~==;:;===l: positive wall, part of vacuum chamber ~ t:t,:==;=f=i=r"iF=;;:=l coaxial copper resonator: hollow cylindrical l upper plasma stream: floating D2 gas feed ~l=t'A'Ft,VtJil/1t:tl't~l=l:~l4..,._ l j magnetic differential coils helical antenna ! glass tube RF generator lower plasma stream: floating Figure 1.5. Schematic of the UIUC RF Gun Injector for IEC Experiments UNCLASSIFIED/ /FOA OFFI51Ak Wlili 0Plk¥ 11 UNCLASSIFIED/ /FOA QFFI&IAL ~!ii: f>HLY 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 fuel, 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-11B 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 light 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, physical 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- 11B 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 UNCLASSIFIED/ /liOA: OlililCI0L: !Pi'li ODIL:¥ 12 UNCLASSIFIED//fOft Offl@IAL l!ISE 8P•tlf 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 Convergent 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 oscillating plasma sphere", Physics of Plasmas 12, (2005) 056315. UNCLASSIFIED/ /FOA OFFIQIAk Wliii 01\lk¥ 13 UNCLASSIFIED/ / FOR OFFIElIAk WSE OPtklf Section II. Select Experiments In this section some select experiments are briefly reviewed with the main focus on ion­ injected IECs. Beginning 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 several 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 0 u 109 w ll) .... ~ 0 Q:: t­ ~ co.I 108 . I­:, 0. 6 107 I­ :J .z 106 .. e- ·--, r, / ' : ~ HIGH VOL.TAG[ DOME CONU,INING [g AUXIL A,tY POWER SUPPI.IES tHGH VOLTAGE INSUL. ATOA 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 109 OT n/s at 150 kV. For Farnsworth perspective, note that, as discussed later, UIUC IEC gridded devices routinely produce 108 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 UNCLASSIFIED/ / &OA: O&&ICI AL !!ii ODt11 X 14 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 ionize) 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 eventually discarded to move to RF guns with much higher beam currents. In addition, the gun design 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 the potential well so that their energy falls 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 the 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 i