Disruption Avoidance via RF Current Condensation in Magnetic Islands Produced by Off-Normal Events

TL;DR

This paper explores how RF current drive can stabilize large magnetic islands in tokamaks, highlighting nonlinear effects like current condensation that improve efficiency but require careful targeting to avoid stabilization hindrance.

Contribution

It introduces the concept of RF current condensation and nonlinear shadowing effects, advancing understanding of stabilization techniques for large magnetic islands in tokamaks.

Findings

RF current condensation concentrates current in island centers

Nonlinear effects can enhance or hinder stabilization

Proper ray trajectory aiming is crucial for effective stabilization

Abstract

As tokamaks are designed and built with increasing levels of stored energy in the plasma, disruptions become increasingly dangerous. It has been reported that 95% of the disruptions in the Joint European Torus (JET) tokamak with the ITER-like wall are preceded by the growth of large locked islands, and these large islands are mostly produced by off-normal events other than neoclassical tearing modes. This paper discusses the use of RF current drive to stabilize large islands, focusing on nonlinear effects that appear when relatively high powers are used to stabilize large islands. An RF current condensation effect can concentrate the RF driven current near the center of the island, increasing the efficiency of the stabilization. A nonlinear shadowing effect can hinder the stabilization of islands if the aiming of the ray trajectories does not properly consider the nonlinear effects.