Lower hybrid current drive at high density in Alcator C-Mod
Author(s)
Harvey, R. W.; Meneghini, Orso-Maria Cornelio; Schmidt, A. E.; Smirnov, A. P.; Wilson, J. R.; Wallace, Gregory Marriner; Hubbard, Amanda E.; Bonoli, Paul T.; Faust, Ian Charles; Hughes, Jerry W.; Labombard, Brian; Parker, Ronald R.; Shiraiwa, Shunichi; Whyte, Dennis G.; Wright, John C.; Wukitch, Stephen James; ... Show more Show less
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Experimental observations of lower hybrid current drive (LHCD) at high density on the Alcator C-Mod tokamak are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops suddenly above line-averaged densities of 10[superscript 20] m[superscript −3] (ω/ω[subscript LH] ~ 3) in single null discharges with large (≥8 mm) inner gaps, well below the density limit previously observed on limited tokamaks (ω/ω[subscript LH] ~ 2). Modelling and experimental evidence suggest that the absence of LHCD driven fast electrons at high density may be due to parasitic collisional absorption in the scrape-off layer (SOL). Experiments show that the population of fast electrons produced by LHCD at high density ([bar over n][subscript e] > 10[superscript 20] m[superscript -3]) can be increased by operating with an inner gap of less than ~5 mm with the strongest non-thermal emission in inner wall limited plasmas. A change in plasma topology from single to double null produces a modest increase in non-thermal emission at high density. Increasing the electron temperature in the periphery of the plasma (0.8 > r/a > 1.0) also results in a modest increase in non-thermal electron emission above the density limit. Ray tracing/Fokker–Planck simulations of these discharges predict the observed sensitivity to plasma position when the effects of collisional absorption in the SOL are included in the model.
Date issued
2011-07Department
Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science; Massachusetts Institute of Technology. Department of Nuclear Science and Engineering; Massachusetts Institute of Technology. Plasma Science and Fusion CenterJournal
Nuclear Fusion
Publisher
IOP Publishing
Citation
Wallace, G.M. et al. “Lower Hybrid Current Drive at High Density in Alcator C-Mod.” Nuclear Fusion 51.8 (2011): 083032.
Version: Author's final manuscript
ISSN
0029-5515
1741-4326