Fast ion driven drift instability in reversed shear plasmas

Abstract

It is shown that electron drift waves can be destabilized by trapped fast ions produced by a fusion reaction because a significant fraction of those particles reverses their precession direction in reversed shear plasmas toward the electron diamagnetic direction and can resonate with the electron drift wave. We perform a local stability analysis and calculate the consequent quasi-linear transport caused by this new instability using bounce-averaged gyrokinetic equations in toroidal geometry and under fusion reactor conditions. We consider the equilibrium distribution function of fast ions as the slowing down distribution and compare with the equivalent Maxwellian case for illustration. The instability occurs when the temperature profile of fast ions peaks sufficiently compared to the density profile, and the magnetic shear is strongly negative. The resulting particle flux of fast ions is outward, while the particle flux of main hydrogenic ions can be inward.

Effects of energetic particles on the density-gradient-driven collisionless trapped electron mode instability in tokamak plasmas

“…Treating thermal ions and EPs in a more realistic toroidal geometry could also be important. The resonance of trapped EPs' precession-driven electron drift instability in strongly reversed magnetic shear plasmas has been found in recent works [30,31]. Nonlinear gyrokinetic simulation addressing other potential effects of EPs on CTEM turbulence might also be important, since relatively short wavelength residual zonal flow enhanced by EPs has been found [35].…”

“…The corresponding thermal ions will become flatter due to the equilibrium quasi-neutrality condition. As shown in equations ( 16)- (30), the normalized real frequency has a positive correlation with the density gradient of thermal ions. Therefore, a steeper density profile of EPs (flatter thermal ions density) results in a stronger downshift of the real frequency and consequent stronger destabilizing effects.…”

“…The latter might also be the origin of the different effects of EPs on CTEM and ITG instabilities. One of the exceptions is for the case with strongly reversed magnetic shear, which is enough to reverse the precession direction of EPs [30,31]. In this work, we will consider the parallel resonance of EPs with the electron drift wave, i.e.…”

“…In this work, both R/L vc and R/L T eff are calculated self-consistently for SD and EM cases, respectively, where R/L T eff obtained for typical parameters is much smaller than that used in [15]. Although this different treatment of the temperature gradient of EPs seems unimportant for CTEM instability driven by the gradient of electron density or temperature, it could be important for ITG [8], or even for electron drift instability driven by fast ion precession in strong reversed magnetic shear [30,31]. In this work, to keep γ ≪ ωr for the validity of our simplified model, we will focus on the long wavelength regime and choose k θ ρ H = 0.15 as the basic parameter in the following.…”

Stabilization of ion-temperature-gradient mode by trapped fast ions

“…The equilibrium distribution function of fast ions is assumed to be a Maxwellian distribution function [11,34]…”