Error Field Assessment from Driven Rotation of Stable External Kinks at EXTRAP-T2R Reversed Field Pinch

TL;DR

This paper introduces a novel non-disruptive method for assessing and correcting error fields in a reversed field pinch device by analyzing the rotation behavior of external kink modes under rotating magnetic perturbations.

Contribution

The study demonstrates a new technique to evaluate and optimize error fields using stable external kink modes, applicable beyond low-density conditions and potentially extendable to tokamaks like ITER.

Findings

Error fields can be accurately inferred from kink mode rotation behavior.

Applying corrections improves discharge duration and mode rotation uniformity.

The method aligns well with theoretical predictions and can be extended to other plasma devices.

Abstract

A new non-disruptive error field (EF) assessment technique not restricted to low density and thus low beta was demonstrated at the EXTRAP-T2R reversed field pinch. Stable and marginally stable external kink modes of toroidal mode number n=10 and n=8, respectively, were generated, and their rotation sustained, by means of rotating magnetic perturbations of the same n. Due to finite EFs, and in spite of the applied perturbations rotating uniformly and having constant amplitude, the kink modes were observed to rotate non-uniformly and be modulated in amplitude. This behavior was used to precisely infer the amplitude and approximately estimate the toroidal phase of the EF. A subsequent scan permitted to optimize the toroidal phase. The technique was tested against deliberately applied as well as intrinsic error fields of n=8 and 10. Corrections equal and opposite to the estimated error fields were applied. The efficacy of the error compensation was indicated by the increased discharge duration and more uniform mode rotation in response to a uniformly rotating perturbation. The results are in good agreement with theory, and the extension to lower n, to tearing modes and to tokamaks, including ITER, is discussed.