Kinetic damping of Alfvén eigenmodes in general tokamak geometry

Abstract

A nonperturbative kinetic/magnetohydrodynamics eigenvalue code has been constructed for calculation of kinetic damping of shear Alfvén eigenmodes in general tokamak geometry with finite pressure. The model describes shear Alfvén waves with kinetic effects from thermal species including thermal ion finite Larmor radius effects and parallel electric field. An analytic formula for the radiative damping of reversed shear Alfvén eigenmodes is obtained for tokamak plasmas with reversed shear q profile. Numerical calculations reveal the existence of multiple kinetic reversed shear Alfvén eigenmodes (KRSAEs). It is found that the damping rate of the KRSAEs scales linearly with the thermal ion gyroradius. The damping rates are larger for modes with more peaks in the radial structures. These results are consistent with analytic expectation. The KRSAEs found here can be used to interpret the RSAEs frequency sweeping down observed sometime in the tokamak experiments.

Combined ideal and kinetic effects on reversed shear Alfvén eigenmodes

“…[16], analytically finds strongly localized kinetic RSAEs (KRSAE) with the radial scale on the order of ρ i , which are potentially stronger damped then the solutions found in our work. Found solutions are similar in localization as ones reported numerically [17] recently. The later reference shows solutions, which are similar to ours when the mode structure becomes narrow.…”

Numerical study of compressibility effects on the reversed shear Alfvén eigenmode in tokamak plasma

“…1 having a minimum of the profile: q min ¼ 1:69, which is more realistic than the flat one in Ref. 21. The single toroidal mode number is n ¼ 4, and the five poloidal mode numbers are m ¼ 5, 6, 7, 8, 9. The specific heat index C is 5/3.…”

“…The code was used to study the kinetic effects on RSAEs including finite parallel electric field and finite Larmor radius. 21 Introducing the compressibility effects in the code, we investigate its effects on the RSAEs in this section. We shall pay special attentions on the variation of the frequency and the existence condition of the modes.…”

“…Recently, the results obtained in NSTX at high beta showed that the compressibility could suppress the RSAEs when the geodesic acoustic frequency is close to or larger than the TAEs frequency. 20 In this work, we consider the effects of compressibility on the RSAEs using the numerical code (KAEC) 21 after incorporating geodesic perturbation. We obtain the following three results.…”

Measurements of the eigenfunction of reversed shear Alfvén eigenmodes that sweep downward in frequency

“…32 This occurs when the ratio of fast-ion gradient to second derivative of the q profile q 00 min is relatively small. A paper by Yu et al 33 shows that the KRSAE eigenfunction is often modulated radially on the ion Larmor radius scale. infernal modes that occur when the q profile is very flat in the core.…”