Stabilization of tearing modes by modulated electron cyclotron current drive

W Zhang,J Zhu,Y Zhang,Z W Ma

doi:10.1063/1.5080379

Abstract

The influence of modulated-ECCD on m/n=2/1 resistive tearing mode is investigated by a three-dimensional toroidal and non-reduced MHD code CLT. It is found that, after applying a modulated-ECCD, tearing mode instabilities are suppressed and magnetic islands are gradually reduced to a low level, then the width of the magnetic islands exhibit periodic oscillation with the time scale of ECCD modulation frequency. The minimum width of magnetic islands decreases with the decrease of ECCD modulation frequency and increases with the increase of the buildup time of the driven current.

Highlights

Neo-classical tearing modes (NTMs)18,19 can be triggered by seed islands which lead to reduction of the bootstrap current inside magnetic islands, resulting in modification of the current profile and reinforcement of initial magnetic perturbations
The influence of modulated-Electron Cyclotron Current Drive (ECCD) on m/n=2/1 resistive tearing mode in EAST is investigated by a three-dimensional toroidal MHD code CLT
We systematically studied roles of modulated-ECCD with different rotation frequencies and current buildup times on m/n=2/1 resistive tearing mode

Summary

INTRODUCTION

Tearing mode instabilities are often observed in tokamak experiments. They are deleterious to tokamak performance as they can reduce core electron temperature, degrade plasma energy confinement or even cause disruptions. Classical tearing modes are driven by unfavorable plasma current density gradient, which gives ∆' > 0 (where ∆' indicates the discontinuity of radial derivative of magnetic perturbation across singular layer). Neo-classical tearing modes (NTMs) can be triggered by seed islands which lead to reduction of the bootstrap current inside magnetic islands, resulting in modification of the current profile and reinforcement of initial magnetic perturbations. ECCD has localized energy and momentum deposition, which enables driven current to localize in magnetic islands.36,37 This method has been widely used to control tearing modes in tokamaks, such as ASDEX upgrade, JT60U,24,38 D-IIID, and EAST.. We know that the driven current deposition around the Opoint of magnetic island leads to a stabilizing effect while the driven current deposition around the X-point results in a destabilizing effect It is found that the efficiency of a continuous driven current is lower than a modulated one (a modulated ECCD means that the driven current deposits only around the O-point of the island), as the driven current around the X-point has a negative effect on suppressing the tearing modes. We carry out a numerical simulation to investigate the influence of modulated-ECCD on tearing modes in EAST by the three-dimensional toroidal non-reduced MHD code CLT.

MODULATED-ECCD MODEL

SIMULATION RESULTS

SUMMARY