Observations of Fast H-Mode Pedestal Dynamics with New MSE/CER Dual-Spectrum Diagnostic at DIII-D

Two novel single-channel Czerny-Turner spectrometers [1,2] have been deployed at DIII-D for simultaneous and co-located measurements of the spectrally resolved Motional Stark Effect (MSE) Beam Emission and the Carbon CVI charge exchange emission at high spectral (∆λ=0.13 nm) and temporal resolution (1-5 kHz). High throughput optics (f/#=2.8) allow for good signal-to-noise at high time resolution using fast EMCCD detectors. Measurements in ITER-similar H-mode plasmas show clear toroidal rotation collapse during ELM crashes, accompanied by Sigma/Pi ratio changes over a similar timescale. At these timescales, non-equilibrium neutral beam populations appear to play a large role in pedestal MSE line ratios, as qualitatively predicted by the NOMAD quantum collisional codebase. Complex neutral beam physics have also been observed and successfully reproduced with a physics forward model, revealing a continuum of beam energies that further complicate the MSE spectrum. Tentative simultaneous fits of the magnetic pitch, non-equilibrium populations, and toroidal velocity will be shown.

[1] Albosta R et al.: Rev. Sci. Instrum. 93, 113546 (2022)

[2] Albosta R et al.: Rev. Sci. Instrum. 95, 093519 (2024)