Plasma response measurements of external magnetic perturbations using electron cyclotron emission and comparisons to 3D ideal MHD equilibrium

TL;DR

This study measures plasma responses to external magnetic perturbations in ASDEX Upgrade using electron cyclotron emission diagnostics, comparing results with 3D ideal MHD models to understand kink responses and mode structures.

Contribution

It provides the first detailed comparison of ECE measurements with 3D ideal MHD simulations of plasma response to magnetic perturbations.

Findings

Measured plasma displacement agrees with VMEC simulations.

Observed kink response amplifies the perturbation beyond vacuum predictions.

Poloidal mode structure peaks at non-resonant mode numbers, consistent with ideal MHD theory.

Abstract

The plasma response from an external n = 2 magnetic perturbation field in ASDEX Upgrade has been measured using mainly electron cyclotron emission (ECE) diagnostics and a rigid rotating field. To interpret ECE and ECE-imaging (ECE-I) measurements accurately, forward modeling of the radiation transport has been combined with ray tracing. The measured data is compared to synthetic ECE data generated from a 3D ideal magnetohydrodynamics (MHD) equilibrium calculated by VMEC. The measured amplitudes of the helical displacement around the low field side midplane are in reasonable agreement with the one from the synthetic VMEC diagnostics. Both exceed the prediction from the vacuum field calculations and indicate the presence of a kink response at the edge, which amplifies the perturbation. VMEC and MARS-F have been used to calculate the properties of this kink mode. The poloidal mode structure of the magnetic perturbation of this kink mode at the edge peaks at poloidal mode numbers larger than the resonant components |m| > |nq|, whereas the poloidal mode structure of its displacement is almost resonant |m| ~ |nq|. This is expected from ideal MHD in the proximity of rational surfaces. The displacement measured by ECE-I confirms this resonant response.