Non-Maxwellian fast particle effects in gyrokinetic GENE simulations

TL;DR

This paper investigates the effects of non-Maxwellian fast particle distributions on plasma turbulence suppression in gyrokinetic simulations, revealing that realistic distributions slightly reduce stabilization but enhance experimental agreement.

Contribution

The study extends the GENE code to incorporate arbitrary background distributions, enabling more accurate modeling of fast ions in gyrokinetic simulations.

Findings

Realistic fast ion distributions slightly weaken turbulence suppression.

Inclusion of non-Maxwellian distributions improves power balance agreement with experiments.

Fast ion effects remain significant even with realistic distributions.

Abstract

Fast ions have recently been found to significantly impact and partially suppress plasma turbulence both in experimental and numerical studies in a number of scenarios. Understanding the underlying physics and identifying the range of their beneficial effect is an essential task for future fusion reactors, where highly energetic ions are generated through fusion reactions and external heating schemes. However, in many of the gyrokinetic codes fast ions are, for simplicity, treated as equivalent-Maxwellian-distributed particle species, although it is well known that to rigorously model highly non-thermalised particles, a non-Maxwellian background distribution function is needed. To study the impact of this assumption, the gyrokinetic code GENE has recently been extended to support arbitrary background distribution functions which might be either analytic, e.g. slowing down and bi-Maxwellian, or obtained from numerical fast ion models. A particular JET plasma with strong fast-ion related turbulence suppression is revised with these new code capabilities both with linear and nonlinear gyrokinetic simulations. It appears that the fast ion stabilization tends to be less strong but still substantial with more realistic distributions, and this improves the quantitative power balance agreement with experiments.