Reconstructions of Global Plasma Oscillations on C-2W via 300-Channel AXUV Photodiode Camera System

In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams (15 – 40 keV, total power up to 20 MW), expander divertors, end bias electrodes, and an active plasma control system. A 300-channel radiation camera system [2] installed on C-2W utilizes AXUV photodiode arrays to collect broadband emission (SXR to NIR) along 180 unique lines of sight that intersect a toroidal plane of the FRC near the mid-plane. Automated scripts process the raw photo-signals after each plasma discharge, yielding time-resolved tomographic reconstructions and centroid trajectories of the near-midplane emissivity profile. Different tomographic algorithms based on Phillips-Tikhonov regularization, minimum Fisher information, and Bayesian maximum entropy are compared [3]. Singular value decomposition (SVD) [4] applied to the reconstructed profiles successfully resolves low-amplitude MHD modes with toroidal numbers n=1 and n=2 within the globally stable plasmas. Both the relative energy of these modes and the centroid wobble amplitude grow substantially after electrode bias termination and during electrode arcing events, indicating decreased plasma stability associated with loss of edge biasing.

[1] H. Gota et al, "Overview of C-2W: High-temperature, steady-state beam-driven field-reversed configuration plasmas", Nucl. Fusion 61 106039 (2021).

[2] A. S. Bondarenko et al, "Tomographic and centroid reconstructions of plasma emission on C-2W via enhanced 300-channel bolometry system," Review of Scientific Instruments 93, 103517 (2022).

[3] M. Anton et al. "X-ray tomography on the TCV tokamak." Plasma Physics and Controlled Fusion 38 (11), 1849 (1996).

[4] T. Dudok de Wit et al, “The biorthogonal decomposition as a tool for investigating fluctuations in plasmas,” Physics of Plasmas 1, 3288 (1994)