Application of the Portable Diagnostic Package to the Wisconsin High-temperature-superconducting Axisymmetric Mirror (WHAM)

TL;DR

This work addresses the need for portable, rapidly deployable diagnostics to validate new confinement concepts across diverse devices. It introduces the Portable Diagnostic Package (PDP), which combines optical emission spectroscopy (OES) for impurity surveys and flow via Doppler shifts with active Thomson scattering (TS) for local electron temperature and density, packaged for cartable deployment. Applied to the Wisconsin HTS Axisymmetric Mirror (WHAM), the PDP achieves simultaneous OES and TS measurements in commissioning plasmas, delivering line-integrated and local impurity/flow data and revealing electron-temperature distributions through TS. The study also details calibration, stray-light mitigation, and timing strategies, highlighting the PDP’s potential to enable multi-parameter, device-agnostic plasma diagnostics and, with improvements, even single-shot TS on higher-density plasmas.

Abstract

We present an application of the Portable Diagnostic Package (PDP) on the Wisconsin HTS Axisymmetric Mirror (WHAM), which integrates an optical emission spectroscopy (OES) system and an active Thomson scattering (TS) system. Due to the designed portability of our system, we realized the installation of the PDP OES and TS measurements on WHAM in $\sim$6 months. The OES system facilitates a comprehensive impurity line survey and enables flow measurements through the Doppler effect observed on impurity lines. Notably, plasma rotation profiles were successfully derived from doubly charged carbon lines. In addition, the TS system enabled the first measurements of the electron temperature in commissioning plasmas on WHAM. These successes underscore the diagnostic package's potential for advancing experimental plasma studies.

Figures (10)

• Figure 1: A schematic illustration of the measurement system. (top) Overview of WHAM: The laser is injected into the central cell on the midplane. The collection optics for the TS and OES systems are attached on the top port. (lower left) Collection optics: The optics consist of a 75 mm achromat and multiple cylindrical lenses, to expand the image along the laser. (middle right) Optical fiber bundle: The light is focused on 3$\times$11 fibers, which are in the bundle. The central row is used for the TS and the bottom row is used for the OES. (bottom right) Image of the optical fibers on the midplane, simulated by an optical ray tracing program. The image is expanded along the laser direction by a factor of $\sim$ 5. The image size of each fiber is $\sim$ 13 mm in the laser direction and $\sim$ 2.7 mm across the laser path.
• Figure 2: (a) CCD image of the doubly charged carbon emission lines, with the horizontal $y$ axis corresponding to the wavelength-dispersion direction and the vertical $x$ axis parallel to the entrance slit. The labeled $X$-positions denote the chords associated with each optical fiber. (b) Extracted line-center positions from each chord (blue markers). The dashed line indicates the line center position based on the nominal calibration, while the solid black line shows the best-fit shift that yields a more symmetric velocity profile. (c) Plasma velocity profile obtained from the line shift as a function of the distance from the plasma axis, $X$. Open markers are the velocity observed in $X < 0$ cast to the positive $X$-side assuming axi-symmetry of the plasma. The velocity profile using the nominal calibration shows strong asymmetry, i.e., the velocity values obtained in the corresponding positions (e.g., +43 mm and -43 mm) are largely different. (d) Plasma velocity profile after applying the correction derived from (b), now displaying a more realistic, near-symmetric shape about the plasma center.
• Figure 3: (a) Photograph of the image intensifier with the 3D-printed optical mask installed. The central opening is $\sim$ 0.6 mm in diameter and mounted off-focus to reduce the intense stray-light halo. (b) Measured stray-light spectra without (black) and with (blue) the mask, showing an overall five-fold reduction and substantially decreased wings caused by intensifier cross-talk. The inset (gray) depicts the mask's triangular transmission profile across the detector plane.
• Figure 4: Overview of shot 250306075, a representative shot in the moderate density $\bar{n_e} = 5.0\times 10^{19}\ {\rm m}^{-3}$ ensemble. The OES system acquired at 5.0 ms, while the TS laser fired at 10.3 ms into the shot, as shown by the vertical dashed lines. (a) The ECH and NBI heating powers. (b) the averaged density from the line-integrated interferometer and the diamagnetic flux measurement from the fluxloop FL1 located at $z=7$ cm.
• Figure 5: Typical OES spectrum around 464--466 nm, where the black dots are the measured data and the gray solid line is the overall fit. The three prominent peaks arise from doubly charged carbon ions (blue vertical bars indicate their rest wavelengths and relative intensities), while the smaller features correspond to singly charged oxygen lines (orange). The spectral profile is obtained by convolving the Doppler broadening from a shifted Maxwellian ion velocity distribution with the asymmetric instrumental function (approximated by a sum of two Gaussians). Fitting this model to the observed spectrum yields the ion density, velocity, and temperature.