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

K. Shih, D. Endrizzi, D. A. Sutherland, J. Anderson, D. Bindl, E. L. Claveau, C. Everson, J. Eickman, S. J. Frank, E. Marriott, E. Penne, J. Pizzo, T. Qian, J. Viola, C. B. Forest, D. Yakovlev

TL;DR

This work introduces the first AXUV-based instability analysis framework for the WHAM axisymmetric mirror, using a 20-channel mid-plane diode array to obtain time-resolved, line-integrated emissivity and derive emission-weighted centroid $\Phi(t)$ and radius $R(t)$. A covariance-based instability metric $\chi$, constructed from the joint dynamics of $(\Phi,R)$, serves as a real-time indicator of macroscopic plasma motion and profile evolution, and is shown to decrease with end-plate bias and anti-correlate with diamagnetic flux during confinement transitions. The analysis relies on geometry-aware data processing, moment definitions, and (optionally) Abel-inversion emissivity reconstruction to validate the physical interpretation of the AXUV signals. The results demonstrate the viability of a compact AXUV diagnostic for real-time stability assessment in magnetic-mirror plasmas and establish a path toward multi-array, energy-resolved extensions for improved spatial coverage and mode tracking.

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 $Φ(t)$ and effective radius $R(t)$. From the joint covariance of these quantities, we define a macroscopic instability parameter $χ(t)$, that quantifies large-scale plasma motion and profile evolution directly from AXUV observables. The parameter $χ$ 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.

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

TL;DR

This work introduces the first AXUV-based instability analysis framework for the WHAM axisymmetric mirror, using a 20-channel mid-plane diode array to obtain time-resolved, line-integrated emissivity and derive emission-weighted centroid and radius . A covariance-based instability metric , constructed from the joint dynamics of , serves as a real-time indicator of macroscopic plasma motion and profile evolution, and is shown to decrease with end-plate bias and anti-correlate with diamagnetic flux during confinement transitions. The analysis relies on geometry-aware data processing, moment definitions, and (optionally) Abel-inversion emissivity reconstruction to validate the physical interpretation of the AXUV signals. The results demonstrate the viability of a compact AXUV diagnostic for real-time stability assessment in magnetic-mirror plasmas and establish a path toward multi-array, energy-resolved extensions for improved spatial coverage and mode tracking.

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 temporal resolution and spatial accuracy across the mid-plane. The data were processed to obtain plasma's statistical moments, yielding time-resolved measurement of the centroid displacement and effective radius . From the joint covariance of these quantities, we define a macroscopic instability parameter , that quantifies large-scale plasma motion and profile evolution directly from AXUV observables. The parameter 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.

Paper Structure

This paper contains 12 sections, 13 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Diagnostic Overview
  3. AXUV Assembly Design
  4. Diagnostic Calibration
  5. Data Analysis Methods
  6. Diagnostic Geometry and Signal Formation
  7. Macroscopic Plasma Parameter Analysis
  8. Summary and Connection to Instability Analysis
  9. Macroscopic Instability Parameter $\chi$ and Experimental Results
  10. Conclusion
  11. Abel inversion and emissivity reconstruction
  12. AXUV Diagnostic Overview Plot

Figures (8)

  • Figure 1: Schematic of the AXUV diagnostic in the WHAM experiment. The blue circular plate represents the AXUV assembly; the pink column represents half of the WHAM plasma; and the orange fans represent the line-of-sight (LOS) trajectories intersecting the plasma.
  • Figure 2: Exploded view of the AXUV assembly. Plasma emission is imaged by the precision gold slit (Layer 1, from the left), passes through the aluminum filter (Layer 3), and is projected onto the AXUV photodiode array (Layer 5). (An intermediate mechanical spacer/support forms Layer 2, 4 and sets the slit–detector distance.)
  • Figure 3: Calibration platform for the AXUV diagnostic. The AXUV assembly is mounted on a rotation and two-axis translation stage, while a fixed laser alignment arm holds the light source. By adjusting the platform orientation and position, the focused laser beam can be directed onto the slit from different lines-of-sight, enabling precise calibration of each diode’s viewing geometry and relative response.
  • Figure 4: Calibration curves for all 20 AXUV diodes, showing the relative response as a function of viewing angle. Each curve corresponds to an individual diode, with colors representing the diode index, where $i = 11 - \text{diode index}$
  • Figure 5: The inter-discharge bias scan, representative $\log(e\,\chi)$ values reconstructed from AXUV data for different end-plate bias settings, shown together with flux-loop stored-energy proxies. The factor $1/e\,\mathrm{mm^2}$ provides a dimensional normalization such that $\log(e\,\chi)$ is dimensionless. Increasing end-plate bias correlates with reduced $\log(e\,\chi)$ and enhanced stored magnetic energy, indicating improved macroscopic stability and confinement.
  • ...and 3 more figures