Thesis Defense

Design and Operation of a Novel Spectrometer Diagnostic at DIII-D for the Measurement of Magnetic Pitch and Toroidal Rotation at ELM-Relevant Timescales

Date: Tuesday, July 22nd

Time: 1:00 pm - 3:00 pm

Place: ERB 106 or

Speaker: Ryan Albosta, Physics PhD Graduate Student

Abstract: Two novel single-channel Czerny-Turner spectrometers [1&2] have been deployed at the DIII-D tokamak for simultaneous and co-located measurements of the full spectrally-resolved Motional Stark Effect (MSE) Split Deuterium 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. Initial results from a 2024 DIII-D ITER-similar H-mode experiment at DIII-D show clear toroidal rotation collapse during ELM crashes from the CER channels, accompanied by Sigma/Pi ratio changes over a similar timescale from the MSE channels. However, non-equilibrium neutral beam excited state 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 has also been observed and successfully reproduced with a physics forward model, revealing a continuum of beam energies that further complicate the MSE spectrum. These effects and their impact on the MSE fitting process during fast ELM dynamics in the DIII-D pedestal will be discussed, as well as the implications for fast magnetic beam-emission measurements – and therefore bootstrap current analysis – in H-mode pedestals generally. Simultaneous magnetic pitch, beam populations, and rotation measurements will be shown during type-1 ELMs at 5 kHz, along with next steps towards further disentangling population and pitch effects within the MSE spectrum during large density fluctuations. [1] Albosta R et al.: Rev Sci Instrum. 2022
[2] Albosta R et al.: Rev Sci Instrum. 2024
*Supported by US DOE grants DE-FG02-89ER53296, DE-FG02-08ER54999 and DE-FC02-04ER54698.

Host: Benedikt Geiger and John Sarff

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