Understanding Non-normal Fluid Behavior in Fusion Plasmas

Abstract

Stable confinement of high temperature fusion plasmas has remained a challenge for the toroidally symmetric magnetic field configuration called “tokamak” for more than half a century. The challenge lies in the fact that improvement of confinement increases the probability of severe fluid instabilities. For instance, the boundary of plasmas in high confinement mode undergoes a recurring cycle of pressure gradient growth due to its effective confinement, followed by an abrupt relaxation that results in a drastic loss of energy and particles. Although a class of well-known eigenmode instabilities are commonly associated with this relaxation, the trigger behind it has remained obscure. To address the trigger problem, we analyzed the dynamical changes around the relaxation events in detail using imaging diagnostics on the KSTAR tokamak and a machine-learning model for event identification. Our research uncovered a more intricate understanding of the fluid dynamics at the plasma boundary, where an eigenmode emerges with the increasing pressure gradient, but is subsequently overridden by a "non-normal" perturbation that quickly escalates and disrupts the confinement. This discovery emphasizes the significance of non-normal fluid behavior at the tokamak plasma boundary and offers valuable insights for improved control of confinement instability in that region.