Plasma potential and energy distributions in high-energy injection machines

Abstract

Steady state conditions are calculated for the Oak Ridge DCX-2 and similar devices in which a plasma confined by magnetic mirrors is maintained continuously by the injection and trapping of energetic ions. The potential difference from plasma interior to exterior is determined in the course of the numerical solution of Fokker-Planck equations for ion and electron energy distributions. It is assumed that the only electron source is ionization of neutrals and trapped ions are lost by charge exchange with neutrals and by Coulomb scattering out of the mirrors. Justification for neglecting cold ions from ionization is discussed. The plasma potential and also, aside from normalization, the energy distributions are found to depend only on the ion injection energy and the fraction of `burn-out' of the neutrals and not directly on absolute magnitudes of neutral pressure or ion injection current. Results are combined with equations governing neutrals to obtain a new DCX-2 performance curve relating injection current to ion density achieved. Enhancement of the charge-exchange loss-rate due to collisional cooling of the ions on electrons, neglected in earlier calculations, is found to increase by a factor of <2 the critical injection current required to reach densities limited only by mirror losses.