Local measurement of error field using naturally rotating tearing mode dynamics in EXTRAP T2R

TL;DR

This paper introduces a novel method to detect and measure intrinsic error fields in a plasma device by analyzing the amplitude modulation of a naturally rotating tearing mode, validated through experiments in the EXTRAP T2R device.

Contribution

The study develops and validates a new technique for error field detection using tearing mode amplitude modulation, including a passive variant without applied RMPs, in a reversed field pinch.

Findings

Successfully identified intrinsic error fields in EXTRAP T2R.

Validated the technique against other error field estimation methods.

Demonstrated the effectiveness of the passive detection method.

Abstract

An error field (EF) detection technique using the amplitude modulation of a naturally rotating tearing mode (TM) is developed and validated in the EXTRAP T2R reversed field pinch. The technique was used to identify intrinsic EFs of $m/n = 1/-12$, where $m$ and $n$ are the poloidal and toroidal mode numbers. The effect of the EF and of a resonant magnetic perturbation (RMP) on the TM, in particular on amplitude modulation, is modeled with a first-order solution of the Modified Rutherford Equation. In the experiment, the TM amplitude is measured as a function of the toroidal angle as the TM rotates rapidly in the presence of an unknown EF and a known, deliberately applied RMP. The RMP amplitude is fixed while the toroidal phase is varied from one discharge to the other, completing a full toroidal scan. Using three such scans with different RMP amplitudes, the EF amplitude and phase are inferred from the phases at which the TM amplitude maximizes. The estimated EF amplitude is consistent with other estimates (e.g. based on the best EF-cancelling RMP, resulting in the fastest TM rotation). A passive variant of this technique is also presented, where no RMPs are applied, and the EF phase is deduced.