Mitigation of sawtooth crash as a manifestation of MHD mode coupling prior to disruption of KSTAR plasma

Abstract

Self-generated n = 1 magnetic perturbation from coupled tearing modes results in mitigation of sawtooth crash, i.e. reduction of sawtooth period and crash amplitude by a factor of several 10 or 100 s (to a few ms and a few or 10 s eV, respectively), which is commonly observed prior to plasma disruption in the KSTAR tokamak. Quasi-3D visualization of the mitigation process shows strong correlations between the sawtooth dynamics and the tearing modes: (1) rapid destabilization of internal m/n = 1/1 mode in the core region followed by incomplete relaxation in the presence of the m/n = 2/1 and 3/1 tearing modes, where the spatial orientation of the relaxation channel is aligned with the displacement direction of core flux surfaces due to the tearing modes, and (2) more reduction of the sawtooth period and amplitude for higher amplitude tearing modes. Application of external n = 1 magnetic perturbation further reduces the sawtooth period and amplitude. The sawtooth mitigation is suggested as an additional disruption warning alarm.