Non-Normal MHD Physics in Fusion Plasmas

Abstract

The simple idea of confining fusion-relevant plasmas using toroidally symmetric magnetic fields called “tokamak” configuration has remained a challenging goal for more than half a century, albeit significant progresses in the plasma physics such as fluid/kinetic instabilities and particle/energy transport processes. One of the major difficulties lies in the fact that improved confinement increases the probability of severe fluid instabilities. In high confinement mode plasmas, the boundary of the plasma is characterized by a semi-periodic growth of steep pressure gradient owing to the “good” confinement provided by the tokamak configuration and its abrupt relaxation leading to a violent loss of energy and particles. It is well known that the relaxation is related to a class of eigenmode instabilities in the steep boundary, but a question remains: What triggers the relaxation? Imaging diagnostics on tokamaks provided a clue to this question, showing that an eigenmode perturbation appears as the pressure gradient increases, and then a “non-normal” perturbation emerges overriding the eigenmode and soon bursts destroying the confinement [J. Lee et al., Scientific Reports 11, 2662 (2021)]. This talk provides an overview on the dynamics of non-normal perturbation as a boundary problem fundamental to the tokamak fusion concept and related wave phenomena [M.H. Kim et al., Nucl. Fusion 60, 126021 (2020)].