Forming and maintaining a potential well in a quasispherical magnetic trap

Nicholas A. Krall
Krall Associates, Del Mar, California 92014

M. Coleman, K. Maffei, J. Lovberg (a), and R. Jacobsen (b)

Directed Technologies Inc., San Diego, California 92121

R. W. Bussard

Energy/Matter Conversion Corp., Manassas Park, Virginia 22111

(Received 18 April 1994; accepted 6 October 1994)

Phys. Plasmas 2 (1), p. 146-158 (Jan. 1995).
(a) Also at ThermoTrex Corporation, San Diego, California 92121.
(b) Also at ARCO Power Technologies, Inc., Washington, DC 20037.

Abstract

The formation and maintenance of an electrostatic potential well by injecting electrons into a quasispherical cusped magnetic confinement geometry is studied experimentally, as a function of plasma fill density and of the energy and current of the injected electrons. A model is developed to analyze the experiment. It is found that the potential is comparable to the energy of the injected electrons at low density, and decreases as an increasing density of cold plasma fills the device because of ionization or wall bombardment. Implications for fusion based on electrostatic/magnetic confinement are discussed.

Section Outline

I. INTRODUCTION
II. THE HEPS [High-Energy Power Source] EXPERIMENT
III. ELECTRON TRAPPING AND POTENTIAL WELL FORMATION — ELECTRONS ONLY
IV. ELECTRON TRAPPING AND POTENTIAL WELL FORMATION — DENSE BACKGROUND PLASMA
A. Experimental data relating to maintenance of the potential
1. Potential versus prefill plasma density
2. Electron density versus time
3. Potential versus injected electron parameters
4. Radial variation of the potential
5. Energy distribution
B. Interpretation of the observations
1. Potential versus prefill plasma density
2. Electron density versus time and prefill density
3. Potential versus injected electron parameters
4. Radial variation of the potential
5. Energy distributions
V. DISCUSSION
ACKNOWLEDGMENTS
APPENDIX: FORMATION AND CONFINEMENT OF A BACKGROUND PLASMA FROM A PREFILL GAS PUFF
1. Experimental results
2. Model

List of Figures

  1. Equatorial flux lines.
  2. Electron beam thermalization (e guns at 5 kV).
  3. Potential profile, electrons only — one electron gun.
  4. Electron density profile calculation.
  5. (a) The dependence of the plasma potential on the initial background plasma density, on a short time scale. (b) The dependence of the plasma potential on the initial background plasma density, on a long time scale.
  6. Potential well behavior 10° off a cusp line during plasma density buildup (no RF).
  7. (a) The time history of plasma density, for nfill = 3*107 cm-3. (b) The time history of plasma density, for nfill = 1.1*107 cm-3. (c) The time history of plasma density, for nfill = 3*108 cm-3.
  8. (a) The dependence of the potential on the hot electron current. (b) The dependence of density on the hot electron current.
  9. (a) The dependence of the potential on the hot electron energy. (b) The dependence of the density on the hot electron energy.
  10. (a) Radial profile of the potential for a low prefill density. (b) Radial profile of the potential for a high prefill density.
  11. Time history of the electron energy distribution.
  12. Potential versus density at early and late time.
  13. Calculated plasma density for plasma prefill = 3*107 cm-3.
  14. Calculated plasma density for plasma prefill = 1.1*108 cm-3.
  15. Typical experimental result for electron density versus time with RF ionization of a prefill background, and no hot electrons.
  16. Time delay for the density rise shown in Fig. 19, versus fill density.
  17. Decay of electron density after the RF is turned off.
  18. Theoretical results for ne(t), varying nfill.
  19. Theoretical results for the time delay, with two different electron confinement models. The experimental results are shown for comparison.
  20. Theoretical results for electron decay after RF shutoff. This compares well with experiment.
  21. Theory of ne(t) when the late influx of wall gas is omitted from the model.

List of Tables

  1. Experimental parameters.
  2. Plasma density at 4.5 ms after the guns are fired.
  3. ne,max and (delay time) versus nfill for variations on the theory.