First implementation of AXUV-based analysis and macro-instability diagnostics on WHAM

TL;DR

This paper presents the first implementation of an AXUV diode array analysis framework on the WHAM device, enabling real-time, quantitative assessment of plasma instabilities through a novel macroscopic instability parameter.

Contribution

It introduces a new AXUV-based analysis method for magnetic mirror plasmas, including a macroscopic instability parameter derived from plasma moments and covariance, demonstrating real-time instability monitoring.

Findings

The AXUV array measures plasma emission with 100 kHz resolution and 1 cm spatial accuracy.

The instability parameter $ppa(t)$ correlates with plasma bias and diamagnetic flux.

The method enables real-time, quantitative instability assessment in magnetic mirror devices.

Abstract

Absolute extreme ultraviolet (AXUV) diode arrays are widely used in fusion experiments for time-resolved measurements of plasma radiation. We report the first implementation of an AXUV-based analysis framework on the Wisconsin High-Temperature Superconducting (HTS) Axisymmetric Mirror (WHAM). A single, precisely calibrated 20-channel AXUV assembly measures line-integrated plasma emission with $ 100~\mathrm{kHz}$ temporal resolution and $\sim1~\mathrm{cm}$ spatial accuracy across the mid-plane. The data were processed to obtain plasma's statistical moments, yielding time-resolved measurement of the centroid displacement $\Phi(t)$ and effective radius $R(t)$. From the joint covariance of these quantities, we define a macroscopic instability parameter $\chi(t)$, that quantifies large-scale plasma motion and profile evolution directly from AXUV observables. The parameter $\chi$ serves as a compact indicator of global macroscopic instability, decreasing with increasing end-plate bias and exhibiting strong anti-correlation with diamagnetic flux during confinement transitions. These results demonstrate that a single AXUV array can provide quantitative, real-time assessment of macroscopic plasma instabilities, constituting the first demonstration of such capability in a magnetic mirror plasma. Future extensions to multiple arrays will further enhance spatial coverage and enable full-mode tracking in axisymmetric mirror configurations and related fusion devices.