Fast particles in drift wave turbulence

J. Weiland, T. Rafiq, E. Schuster

Physics of Plasmas 30, 042517 (2023)

Abstract

This study aims to incorporate the effects of fast particles into our present fluid model for tokamak transport. The parameter epsilon_f=omega/omega_f , where omega is the mode frequency and omega_f is the typical frequency of the fast particles, which enters as a factor in front of the fast particle response. Thus, for trapped fast particles, where omega_f=omega_pres the precession frequency of the fast particles, this parameter is of order 10^-2 for drift waves, and thus, the fast particle response can be neglected. However, epsilon_f will be of order 1 for fast particle modes such as in the fishbone instability. An important turbulence property, affecting both these limits, is resonance broadening. Effects of resonance broadening have recently been considered for fast particle instabilities, often coupled directly to the linear growth rate, while we here consider the original Dupree formulation where the turbulence directly drives a nonlinear frequency shift. Resonance broadening has a general tendency to counteract dissipative wave particle resonances. This has been observed for fast particle instabilities. Here, there is a resonant external source for the fast particles, so the instability survives if this source is dominant over the resonance broadening. For drift waves, however, external sources are not resonant since epsilon_f<< 1. Thus, the resonance broadening is able to remove the dissipative wave particle resonance completely.