Suppression of Tearing Modes by RF Current Condensation

TL;DR

This paper investigates how nonlinear feedback mechanisms involving temperature perturbations can enhance rf current drive efficiency in stabilizing tearing modes in tokamaks, with implications for ITER.

Contribution

It introduces the concept of rf current condensation, showing how nonlinear temperature effects can improve stabilization and reduce sensitivity to misalignment.

Findings

Nonlinear feedback can cause bifurcation in temperature perturbations.

Rf current condensation enhances stabilization efficiency.

The threshold for the effect is relevant for current experiments and ITER.

Abstract

Currents driven by rf (radio frequency) waves in the interior of magnetic islands can stabilize deleterious tearing modes in tokamaks. Present analyses of stabilization assume that the local electron acceleration is unaffected by the presence of the island. However, the power deposition and electron acceleration are sensitive to the perturbation of the temperature. The nonlinear feedback on the power deposition in the island increases the temperature perturbation, and can lead to a bifurcation of the solution to the steady-state heat diffusion equation. The combination of the nonlinearly enhanced temperature perturbation with the rf current drive sensitivity to the temperature leads to an rf current condensation effect, which can increase the efficiency of rf current drive stabilization and reduce its sensitivity to radial misalignment of the ray trajectories. The threshold for the effect is in a regime that has been encountered in experiments, and will likely be encountered in ITER.