Effect of Charge Exchange on the Critical Current for the Onset of Ion-Wave Instabilities

Abstract

Recent experimental results on the afterglow in the stellarator indicate that upon passage of an electrical current through the plasma rapid diffusion sets in at a certain critical current. This critical current can be computed theoretically and is found to depend in significant fashion on the tail of the ion distribution function. This tail may well be non‐Maxwellian due to charge exchange with neutrals. The ion distribution function is computed by solving the Fokker‐Planck equation, and it is found that the tail may be suppressed if there are sufficient neutrals present. This distorted distribution is then used to predict a new critical current for ion‐wave instabilities. The calculations are presented in a form which admits of easy comparison with experiment.