UNIVERSITY OF CALIFORNIA Lawrence Radiation Laboratory Berkeley and Livermore, California Contract No. W-7405-eng-48 UCRL-10852 UC-20 Controlled Thermonuclear Processes TID-4500 (19th Ed.) CONTROLLED THERMONUCLEAR RESEARCH SEMIANNUAL REPORT January through June 1963 July 22, 1963 CONTENTS INTRODUCTION - 1 I. PYROTRON (MAGNETIC MIRROR) PROGRAM 1. Introduction and Summary (Post) - 5 2. Multistage High-Compression Experiments (Coensgen, Cummins, Ellis, Nexsen, and Sherman) - 7 3. Table Top III (Perkins and Barr) - 11 4. Alice (Energetic Neutral-Atom Injection) (Damm, Foote, Futch, Gardner, Gordon, Hunt, Steinhaus, Post, and Oleson) - 19 II. ASTRON PROGRAM 1. Introduction (Christofilos) - 36 2. Astron Electron Accelerator (Hester, Lamb, Reagan, Sherwood, Spoerlein, and Wright) - 37 3. Current Distribution of the E Layer (Christofilos) - 37 III. LIVERMORE PINCH PROGRAM (Birdsall, Colgate, Furth, Hartman, Munger, and Spoerlein) 1. Levitron - 45 2. Linear Hard-Core Pinch - 49 3. Collisionless Plasma Shock - 56 4. Instability Theory - 56 IV. BERKELEY PLASMA RESEARCH 1. Rotating-Plasma Research (Halbach, Layman, Paxson, and Ehlers) - 59 2. Hydromagnetic Waves and Ion Cyclotron Heating (Hamilton, Wilcox, Boley, Cooper, and Spillman) - 62 3. Hydromagnetic Ionizing Fronts (Sherwood and Kunkel) - 85 4. On the Heating of Electrons in Weakly Ionized Gases by Applied Electric and Magnetic Fields (Kunkel and Pearson) - 86 5. Sheet Pinch Studies (Anderson) - 88 6. Experiments on Lorentz Ionization, Gas Collisional Excitation and Electron Detachment, and Diffusion of Partially Ionized Plasmas (Pyle and co-workers) - 91 7. Thermal Instability of the Glow Column (Ecker) - 96 V. THEORETICAL AND BASIC EXPERIMENTAL PLASMA PHYSICS 1. A Self-Consistent Distribution of Electrons in the Astron E Layer (Heckrotte and Neil) - 99 2. Computational Study of Electron Injection in the Astron Device (Neil and Heckrotte) - 100 3. Statistical Derivation of Hydrodynamics for a Coulomb System (Kaufman) - 103 4. The Interchange Instability (Newcomb) - 105 5. Lorentz Ionization Studies (Bailey, Hiskes, and Tarter) - 105 6. Magnetic Shocks (Zwick and Gonzales) - 106 7. The Boltzmann Equation for a Plasma (Geesaman) - 109 8. Statistical Theory of Transport Phenomena (Hall) - 109 9. Theoretical Examination of the Oak Ridge PTF Experiments (Hall) - 110 10. Stability of Longitudinal Oscillations in Anisotropic Plasma (Kammash and Heckrotte) - 111 11. Vertical Instability of the Beam in a Particle Accelerator (Neil and Sessler) - 112 12. Stability of an Inhomogeneous Plasma (Woods and Kammash) - 114 13. Bumpy Torus (Gibson, Jordan, and Lauer) - 116 14. Atomic Scattering and Cross-Section Measurements (Brink, Chambers, and McFarland) - 123 VI. ENGINEERING AND TECHNOLOGICAL DEVELOPMENT 1. Ultrahigh-Vacuum Development (Milleron) - 128 2. Mechanical Engineering Development (Batzer) - 128 3. Electrical Engineering Development (Van Ness) - 135 VII. TALKS AND PUBLICATIONS - 142 Some of the more significant results and conclusions of the past six months in controlled thermonuclear research are briefly described below. The continued success of the finite-Larmor-orbit and finite-conductivity theories of plasma behavior in providing explanations of experimental results and guidance for new plasma experiments is most encouraging. Considerable progress has been made toward completion of new experimental facilities and in modifying existing facilities to meet the need for the attainment of higher plasma temperature and longer confinement time. Toy Top: In the multiple-compression experiments it was reported previously that neutron production is observed for about 250 microseconds, with the maximum occurring a few microseconds before the maximum in the magnetic field and the rate falling off with a 1/e time constant of 50 to 70 μsec. From observation of the signals on arrays of probes detecting charged particles escaping axially beyond the magnetic mirror and energetic neutral particles ejected radially in the median plane of the final compression stage, it is concluded that the decay of the plasma is predominantly caused by charge exchange on contaminants and not by instabilities. This is a most encouraging result. Neutral yield curves have been completed for the compression cycle on the assumption of the presence of background neutron gas at various pressures. The results are in good agreement with the experimental yield curve and indicate that the residual gas molecular density is in the range of about 2 to 5 x 10^11 cm^-3. A vigorous effort is now under way to determine the sources of contaminants and to reduce or delay their effects on the plasma. Operations are soon to be shifted to the larger 2X facility, in which the better vacuum design and greater aperture are expected to contribute to a reduction and delay of the contaminants. Alice: Plasma oscillation of frequency close to the ion precessional frequency had previously been reported in the Alice energetic neutral atom injection facility with beam intensity up to about 60 mA, residual gas pressure of about 10^-8 torr, and density of energetic (20-keV) captured ions of about 10^8 cm^-3. These oscillations are believed to be of the stable type predicted by the finite-orbit theory. When the particle beam is turned off two modes of plasma decay are observed, one in which the frequency of the oscillations decreases as the plasma density decreases, and the other in which the frequency remains constant as the plasma decays. These two modes are now believed to correspond to two branches of the curve for stable oscillations predicted by the theory. Analysis of the buildup and decay of the Alice plasma leads to the conclusion that not more than 20% of the beam particles captured as ions are ionized by collisions with residual gas molecules, the remaining 80% or more being ionized from highly excited states by the Lorentz v x B force. Since the residual gas pressure of 10^-8 torr is still about ten times as high as would permit exponential buildup of the plasma to much higher density, the Alice facility is now undergoing major changes which should reduce the base pressure to 10^-9 torr. If this is indeed the case, the density of captured energetic ions should then increase to 10^9 ions/cm^3 or more, at which--according to the finite-orbit theory--the plasma density should be above the predicted range of flute instability and into the upper stable regime. Table Top: In the slow magnetic compression experiments in the Table Top machine the field configuration has been changed from the simple mirror field, with field intensity maximum at the ends and minimum at the median plane, to one in which there is a secondary maximum in the median plane. The ratio of the magnetic field strength at the central plane to that at the field minimum was about 1.26 at the axis and 1.06 at the chamber wall. At low plasma density the rotational instabilities in the two lobes of the field were found to be entirely uncorrelated. However, as the density was increased approaching the stable regime, the incidence of correlated behavior (indicating a degree of coupling across the central mirror) was observed to occur in an increasing proportion of the observed events. Astron: The Astron electron accelerator has continued in operation with increased pulsed beam current of about 120 amperes at 4.1 MeV. The self-consistent solution for the E layer previously reported has been modified to allow not only the current density, but also the axial derivative of the density, to go to zero at the end of the E layer. This modification is also consistent with programming the currents in the field coils in such a way that the field due to the E layer is entirely contained within the present array of field coils as the E layer is built up, so that no additional end coils are required. All the necessary power supplies for energizing the field coils are scheduled for delivery within the next few weeks, and construction is proceeding so that injection of electrons in experiments directed toward the buildup of the E layer are expected to begin some time during the coming fall. Levitron: In the operation of the Levitron the development of a low-amplitude instability mode, beginning about 200 μsec after the start of the cycle, was previously reported. Some characteristics of this instability have since been determined by carrying out correlation probe measurements along and across the helical magnetic field lines. The observations reveal a high correlation along the field lines, with correlations in radial direction as well. The wave frequency is low compared with that of Alfvén waves, and the wavelength rather short. These observations help to define the character of the instability but as yet do not permit a specific classification. Bumpy Torus: The injection section of the Bumpy Torus has been set up with a 150-keV electron gun. The performance of the split-magnet section in guiding the injected electrons into the interior of the torus, where they spill out in both directions of the toroidal field from the split magnet, has been observed and is found to be substantially as predicted. Homopolar V: The stilbene crystal and pulse-discriminator circuit for observing fast neutrons (> 1 MeV) in the presence of x rays has been completed and used to determine the time history of neutrons from the Homopolar V device. During the first few microseconds after application of the voltage there is a large burst of neutrons due to bombardment of the central electrode, which is normally at negative potential. This initial burst saturates the electronics of the discriminator circuit so that observations cannot be made during the first few microseconds of the discharge. However, after recovery of the detector circuit, neutrons are observed for at least 150 μsec after initiation of the discharge, indicating that energetic deuterons are confined for at least that length of time. The Homopolar Gun: This is a single-ended rotating-plasma device in which the plasma is produced in the tapered field region near a magnetic mirror and is eventually to be ejected into a region of lower magnetic field along the field lines. This device has been operated initially with a second mirror to stop and confine the plasma. After some modification necessitated by some unexpected features of behavior, the device has been performing qualitatively as intended. A version of the homopolar gun is being made for use on the Alice machine to provide an initial pulse of dense plasma of high purity to be injected into the Alice machine from one end as a target for ionizing the atoms of the energetic neutral beam. Sheet Pinch: In a tubular sheet pinch device (Triax) designed specifically to permit end-on viewing of the plasma cylinder, the development of the tearing mode of instability predicted by the finite-conductivity theory has been demonstrated. End-on photographs taken with a Kerr-cell framing camera show the bunching of the plasma cylinder into a multiplicity of strands initially parallel with the axis of the tube and then later twisted into a spiral form. A detailed experimental study of the tearing mode of instability can now be carried out and the results compared with theoretical predictions. 1. INTRODUCTION AND SUMMARY (Richard F. Post) During the period covered by this report additional experimental and theoretical insight was gained concerning the plasma confined in the Alice experiment, and a significant conclusion was reached regarding charge-exchange losses in the multistage experiment. Some informative experiments with modified mirror fields were performed in the Table Top facility, and a further extension of the finite-orbit theory was made and employed in the interpretation of the Alice results. In the multistage experiment exhaustive measurements were made of the radial dependence of the time-resolved flux of ions escaping axially from the compressed plasma column. The results confirmed the elimination of gross transverse plasma drift and showed that the decay in neutron emission is entirely explainable in terms of charge-exchange losses from gas evolved by chamber wall bombardment. Furthermore, modes m < 4 do not develop appreciably, in agreement with finite-orbit stabilization. 2. MULTISTAGE HIGH-COMPRESSION EXPERIMENTS (Toy Top III S and 2X) Frederic H. Coensgen, William F. Cummins, Robert E. Ellis, William E. Nexsen, Jr., and Arthur E. Sherman Investigations in Toy Top III S focused on reaction history and transverse plasma drift. Transverse drift velocity (m=1 flute instability) has an upper limit of 10^4 cm/sec. Construction of the 2X facility has reached 90% completion, with initial pulsed operation anticipated in October. 3. TABLE TOP III Walton A. Perkins and William L. Barr Stability studies with improved vacuum conditions (base pressure ~10^-9 torr) and molybdenum-gettered walls demonstrated that off-axis plasma column rotation could persist for hundreds of milliseconds as it slowly abraded itself at the wall. Experiments in a double-mirror geometry showed correlation of rotations across the central bump at higher plasma densities. 4. ALICE (ENERGETIC NEUTRAL-ATOM INJECTION) Charles C. Damm, James H. Foote, Archer H. Futch, Jr., Andrew L. Gardner, Frank J. Gordon, Angus L. Hunt, James F. Steinhaus, Richard F. Post, and Norman L. Oleson Observed density transitions and low-frequency oscillations match extended finite-orbit stabilization theory. Diagnostics and ion source developments, including cold plasma target injection and Lorentz ionization measurements, were advanced. 1. INTRODUCTION (Nicholas C. Christofilos) The Astron electron accelerator reached operational status at 4 MeV (with up to 120 A pulsed current). Major focus is centered on the buildup of the E layer and avoiding resonant coupling to cavity microwave modes or ion plasma waves. 2. ASTRON ELECTRON ACCELERATOR Ross E. Hester, William A. S. Lamb, Daryl D. Reagan, William A. Sherwood, Ross L. Spoerlein, and Robert E. Wright Peak currents of 150 A were achieved at 4 MeV ± 100 keV. Studies on beam propagation through gas-filled cells and Mylar foils are progressing. 3. CURRENT DISTRIBUTION OF THE E LAYER N. Christofilos A self-consistent current distribution was formulated requiring no end coils by ensuring that the radial component of the E-layer self field and its vector potential vanish at the layer ends: j(z) = sum [ c_n (1 - cos(nkz)) ]. 1. LEVITRON Dale H. Birdsall, Stirling A. Colgate, Harold P. Furth, Charles W. Hartman, Richard H. Munger, and Ross L. Spoerlein Following overhaul and simplified coil installation, slow-field inverse-pinch coupling (2 msec) was explored. Low-amplitude magnetic disturbances (5 G out of 5000 G) correlating with containment loss at 200 μsec were analyzed. Hollow-cathode plasma injection tests were conducted. 2. LINEAR HARD-CORE PINCH Electrode sheath effects and the launching and propagation of torsional Alfvén waves were investigated. 3. COLLISIONLESS PLASMA SHOCK Theoretical mass ratio 100 calculations and experimental collisional heating measurements were concluded. 4. INSTABILITY THEORY Variational energy principles for finite-resistivity hydromagnetic instabilities (tearing, rippling, gravitational modes) and velocity-space Harris instabilities in the E layer were developed. 1. ROTATING-PLASMA RESEARCH Homopolar V neutron histories and Homopolar Gun I design/operation were investigated. 2. HYDROMAGNETIC WAVES AND ION CYCLOTRON HEATING Ion cyclotron resonance experiments, spectroscopic observations of decaying hydrogen plasmas in Hothouse I, two-mode hydromagnetic excitations, and plasma surface waves were measured. 3. HYDROMAGNETIC IONIZING FRONTS Sherwood and Kunkel studied rotating ionization spokes and front propagation. 4. ELECTRON HEATING IN WEAKLY IONIZED GASES Kunkel and Pearson analyzed electron velocity distributions in crossed E x B fields. 5. SHEET PINCH STUDIES Anderson investigated tearing-mode instabilities in Triax discharges. 6. LORENTZ IONIZATION AND DIFFUSION EXPERIMENTS Pyle and collaborators measured excited-state cross sections, ac positive column instabilities, and line source diffusion. 7. THERMAL INSTABILITY OF THE GLOW COLUMN Ecker formulated the theory of m=0/m=1 thermal instabilities in collision-dominated columns. Theoretical studies included: - Self-consistent Astron E-layer electron distribution (Heckrotte and Neil) - Computational study of electron injection on LARC (Neil and Heckrotte) - Statistical derivation of hydrodynamics for Coulomb systems (Kaufman) - Finite Larmor radius effects on interchange instability (Newcomb) - Lorentz ionization of excited hydrogen (Bailey, Hiskes, Tarter) - Symmetric overlapping loops in magnetic shocks (Zwick and Gonzales) - Green's function approach to the Boltzmann equation (Geeseman) - Statistical theory of transport phenomena (Hall) - Model for the Oak Ridge PTF plasma (Hall) - Longitudinal oscillations in anisotropic plasma (Kammash and Heckrotte) - Vertical instability of stacked accelerator beams (Neil and Sessler) - Guiding-center stability of inhomogeneous plasma (Woods and Kammash) - Bumpy Torus electron injection and adiabatic drift orbits (Gibson, Jordan, Lauer) - Charge-transfer and electron emission measurements (Brink, Chambers, McFarland) 1. ULTRAHIGH-VACUUM DEVELOPMENT (Norman Milleron) Optimization of oil boiler cracking, two-stage 5-in. diffusion pumps, bakeable valve traps, and PIG discharge leak detectors. 2. MECHANICAL ENGINEERING DEVELOPMENT (Thomas H. Batzer) Progress on the 2X facility assembly, Alice magnet rebuilding, Astron 720-kV electron gun and cantilever coil alignment, Bumpy Torus components, Levitron cryogenic pumping, and sodium cryogenic coil testing. 3. ELECTRICAL ENGINEERING DEVELOPMENT (Hugh W. Van Ness) Power systems and controls for Table Top III, Toy Top III S, 2X capacitor banks (10 MJ total capacity), Alice diagnostic instrumentation, Levitron ignitron crowbars, Astron core pulsing systems, and switch development (WX 4973 ignitrons). Lists seminars, conference presentations, and journal publications by laboratory staff appearing in Annals of Physics, Applied Optics, The Physics of Fluids, The Physical Review, Physical Review Letters, and Le Vide during the reporting period.