The Effect of Externally Applied 3D Fields on NSTX Edge Turbulence

TL;DR

This study investigates how externally applied 3D magnetic fields influence edge turbulence in NSTX H-mode plasmas, finding minimal impact on turbulence characteristics but confirming localized flux surface displacements consistent with magnetic modeling.

Contribution

It provides the first detailed comparison of edge turbulence before and after applying non-axisymmetric magnetic perturbations in NSTX, validating magnetic displacement predictions.

Findings

3D fields do not significantly alter turbulence size or fluctuation levels.

Radial displacement of turbulence correlates with magnetic field predictions.

Localized flux surface shifts are observed and modeled accurately.

Abstract

We report on a study of the structure of edge turbulence in NSTX H-mode discharges with applied n=1 and n=3 non-axisymmetric magnetic perturbations. The edge turbulence is diagnosed in NSTX using the gas puff imaging (GPI) system to understand how these 3D fields affect edge transport. The presented database study uses a selection of discharges that have a change in the RWM coil current during the GPI gas puff. We compare the turbulence before and after this change in the applied non-axisymmetric fields, and compares the turbulence between the two states. Our findings show that these 3D fields do not have a strong, statistically significant impact on the average poloidal size, autocorrelation time, or relative fluctuation levels of the turbulence. In addition, we find that the displacement of the radial location of the GPI signal peak intensity is correlated with the shift in the perturbed separatrix location as predicted by the 3D vacuum magnetic field code MAFOT. We find that in general the non-axisymmetric fields from the RWM coils locally displace the edge flux surface as expected even when the self-consistent plasma response is not included, and that the model reproduces the localized shift observed by GPI.