Fishbone instability driven by trapped fast ions in a toroidal plasma with reversed magnetic shear

Abstract

Based on the non-perturbative approach, the hybrid code MARS-K is applied to study fishbone (FB) instabilities driven by trapped fast ions in a toroidal plasma with q profile nearly being flat or non-monotonic. We explore the dependency of the FB with variation in the fast ion distribution, thermal particle kinetic effects, safety factor (q) profile and plasma resistivity. When the safety factor minimum value is larger than unity (i.e. ), the mode can be excited by isotropic or anisotropic fast ions, with the latter strongly enhancing the mode growth rate. The mode frequency increases with increasing , and is more easily triggered in a equilibrium with two q = 1 surfaces, compared with the case with one or no q = 1 surface. Kinetic contributions from transit resonance of passing fast ions and from bounce resonance of trapped fast ions strongly enhance mode instability. The passing thermal ions induced Landau damping has a strong stabilization effect. Furthermore, in such a weak, even reversed magnetic shear plasma, the radial mode structure of FB mode monotonically decreases to zero at q = 1 flux surface instead of a step-like function, and it depends on the kinetic contributions from particles. In addition, the plasma resistivity significantly stabilizes the mode near the marginally unstable point.

Synergistic Influences of Kinetic Effects from Thermal Particles and Fast Ions on Internal Kink Mode

“…As discussed in Ref. [19], for the self-consistent computations, the IK instability satisfies the general dispersion relation. The normalized version of the general dispersion relation can be rewritten as 𝜔 2 𝑟 − 𝛾 2 = Re(𝛿 ŴMHD + 𝛿 ŴK ) and 2𝜔 𝑟 𝛾 = Im(𝛿 ŴMHD + 𝛿 ŴK ).…”

“…[10−17] Moreover, the kinetic effects from thermal particles modify the inertia and the perturbed potential energy of the MHD mode, [18] and also modify the the threshold value for driving fishbone mode. [19] However, the kinetic effect of thermal particles on the internal kink (IK) mode requires further investigations. Therefore, in this work, we carry out this kind of study, and investigate the synergistic influence of thermal and fast particles on IK.…”

Effect of anisotropic fast ions on internal kink stability in DIII-D negative and positive triangularity plasmas

“…The detailed MARS-K formulation is presented in reference [50]. The MARS-K code has been successfully benchmarked against other codes [55,56], and extensively applied to study drift-kinetic effects on the internal kink modes [15,20,27,37,57], and external kink/resistive wall modes [36,50,[58][59][60][61][62][63][64][65] in both positive and negative triangularity plasmas. The hybrid model in the MARS-K code utilizes a pressure-coupling scheme, where the fast-ion drift kinetic effect is coupled to the bulk plasma equation through the pressure tensor in the momentum equation as shown below.…”