Simple ID Fokker-Planck modeling of ICRF heating at arbitrary harmonics accounting for non-Maxwellian plasma populations

摘要

INTRODUCTION To rigorously assess the impact of ion cyclotron resonance frequency (ICRF) heating on the distribution functions of the various plasma constituents, a set of coupled Fokker-Planck equations needs to be solved, ideally in conjunction with a powerful wave equation solver and a transport code. As doing so while accounting for all details of the wave-particle interaction is time consuming, simpler models are often used to get a crude impression of how the absorbed RF power affects the plasma. Stix[1,2] proposed a, method to analytically compute the isotropic (pitch angle averaged) distribution function of a population heated by electromagnetic waves. To ensure that the solution can be found in analytical form, the applicability of the expression provided by Stix was somewhat limited: It was assumed that the particles are heated at their fundamental cyclotron frequency and that a not too energetic minority tail is formed. Moreover, the background plasma particles were assumed to be Maxwellian. Allowing for numerical rather than analytical integration and adopting the general Coulomb collision operator for arbitrary distributions proposed by Karney [3], Stix's method can immediately be extended to describe ICRF heating of not -only small minorities but also of large populations at any cyclotron harmonic, fully accounting for their Coulomb collisional interaction by solving a set of coupled Fokker-Planck equations in which none of the species is assumed to be Maxwellian.