The distortion of the thermal-ion distribution during neutral-injection heating

Abstract

It is shown that a high-energy tail may form on the background-ion distribution during neutral-injection heating. The temperature of this tail is found to depend critically upon the velocity dependence of the particle and energy loss mechanisms. – The Fokker-Planck equation for the equilibrium thermal-ion distribution is solved both numerically for all v and analytically for large v. Results from the numerical solution are presented for various cases of particle or energy loss including particle loss by charge exchange or diffusion and energy loss by thermal conduction. An analytic expression for the ratio of tail to actual temperature is derived for the case of good high-energy containment and for energy-independent particle loss (e.g. charge exchange). The expression derived for the latter case is shown to be in good agreement with the numerical results.