An active feedback recovery technique from disruption events induced by m=2 n=1 tearing modes in ohmically heated tokamak plasmas

TL;DR

This paper demonstrates a novel active feedback technique in tokamak plasmas that converts disruptive tearing modes into benign modes by controlling the edge safety factor q(a), significantly improving plasma stability.

Contribution

A new combined feedback method controlling q(a) and tearing modes to prevent disruptions in ohmically heated tokamaks.

Findings

Successful conversion of tearing modes to resistive wall modes in experiments.

Achieved 100% success rate in preventing disruptions under tested conditions.

Proposed control of q(a) via plasma shape adjustments for future tokamaks.

Abstract

We present experimental results of magnetic feedback control on the m=2, n=1 tearing mode in RFX-mod operated as a circular ohmically heated tokamak. The feedback suppression of the non-resonant m=2, n=1 Resistive Wall Mode (RWM) in q(a)<2 plasmas is a well-established result of RFX-mod. The control of the tearing counterpart, which develops in q(a)>2 equilibrium, is instead a more difficult issue. In fact, the disruption induced by a growing amplitude m=2, n=1 tearing mode can be prevented by feedback only when the resonant surface q=2 is close to the plasma edge, namely 2<q(a)<2.5, and the electron density does not exceed approximately half of the Greenwald limit. A combined technique of tearing mode and q(a) control has been therefore developed to recover the discharge from the most critical conditions: the potentially disruptive tearing mode is converted into the relatively benign RWM by suddenly decreasing q(a) below 2. The experiments demonstrate the concept with 100% of successful cases. The q(a) control has been performed through the plasma current, given the capability of the toroidal loop-voltage power supply of RFX-mod. We also propose a path for controlling q(a) by acting on the plasma shape, which could be applied to medium size elongated tokamaks.