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Title 479: Thermal Energy Scan for Helium MGI
Name:John C. Wesley () Affiliation:General Atomics
Research Area:Disruptions Presentation time: Not requested
Co-Author(s): Team MGI
Description: Previous experiments with argon MGI using the Mark IV 'directed jet' injection system demonstrate a significant increase in assimilated ion and electron content with increasing plasma thermal energy (W_th). Experiments with helium MGI using the 5-valve MEDUSA injector show high assimilation, ~30%, with W_th = 0.7 MJ ELMy H-mode plasmas. Increase in W_th is expected to increase assimilation. A 2-2.5 MJ target plasma will be more 'ITER-like' with regard to reaching a high Rosenbluth density fraction in terms of the ratio of W_th to added He and electron content. The basic concept will be to do 6-valve short-pulse He MGI into a maximum W_th 'ITER-like' ELMy H-mode plasma with 7-source NBI heating. Supplemental ECH and/or ICH is an option. Need to assess whether incremental benefit is significant.
Experimental Approach/Plan:
Background: Assimilation of injected neutral gas in MGI depends on plasma thermal energy. Past data and experience + theories indicate positive W_th dependence of assimilation fraction. With regard to achieving total electron densities = Rosenbluth density (~4e22 m-3), all present tokamaks, DIII-D included, have lower ratio, at 'maximum performance', of W_th/n_RB than ITER will have. Hence experiments with maximum W_th are needed to better approach 'ITER-like' conditions and elucidate W_th scaling in the otherwise most efficient short-pulse MGI (MEDUSA) regime.
Resource Requirements: q ~ 3 'ITER-like' ELMy H-mode target with 1->7 source NBI, plus supplemental ECH or ICH if warranted and available. Option: 'advanced baseline' hybrid target rather than standard sawtoothing H-mode. MEDUSA six-valve injector, helium gas. Standard MGI/disruption diagnostics, especially fast magnetics, 4-chord fast C02 interferometer, DISRAD + TS etc. for before-injection characterization. Shot plan will have to accommodate need (~ 30 min) to recycle NBI cryopumping after each shot. Consider scenarios for a dedicated experiment day (or half day) with ca 4 MGI shots vs. four end-of-day experiments
Diagnostic Requirements: See Resource. Also spectroscopic and/or probe measurements of current quench plasma attributes.
Analysis Requirements: Characterization of pre-injection target; eventual modelling with 2-D codes and diagnostic simulations to interpret C02 data, etc.
Other Requirements: Similar He MGI into a high-performance 'AT' plasma would be interesting. Need to examine possible options for 'end-of-day' piggyback tests on a variety of high-performance plasmas. Some expectation that MGI will affect subsequent target plasma, so 'during-day' piggybacks may not be advisable.