Chirping modes in hybrid tokamak discharges

Abstract

In the so-called “hybrid” shots characterized by shear-free core with safety factor slightly above 1, an interesting yet unexplained chirping activity has been observed in tokamaks with a high fast ion content. Namely, fishbone modes with m=n=1 and downward frequency chirping are accompanied by frequency jumps of the tearing modes with m=n+1 and n > 1 [here m(n) is the poloidal (toroidal) mode number]. In the present work, these related phenomena are attributed to a combination of the two impulsive toroidal torques: (1) recoil torque applied to the shear-free core during fishbone burst and (2) electromagnetic torque applied to the tearing layer. We have developed a quasi-linear model in which fast transients of the plasma toroidal rotation induced by these torques qualitatively reproduce the observed chirping dynamics.