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J.W. Hughes, A. Hubbard, B. LaBombard, J. Terry, W. Rowan, R. McDermott C-Mod Ideas Forum

Continued exploration of pedestal structure and edge relaxation mechanisms at lower edge collisionality . J.W. Hughes, A. Hubbard, B. LaBombard, J. Terry, W. Rowan, R. McDermott C-Mod Ideas Forum December 13, 2006.

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J.W. Hughes, A. Hubbard, B. LaBombard, J. Terry, W. Rowan, R. McDermott C-Mod Ideas Forum

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  1. Continued exploration of pedestal structure and edge relaxation mechanisms at lower edge collisionality  J.W. Hughes, A. Hubbard, B. LaBombard, J. Terry, W. Rowan, R. McDermott C-Mod Ideas Forum December 13, 2006

  2. Motivation: Low-n* pedestal regimes make contact with other devices, relevant to ITER • H-mode pedestal height critical for determining core profiles, confinement in ITER (or any fusion device) • Majority of work done elsewhere on pedestal and confinement scalings performed in ELMy regimes (pedestal n*<1) • C-Mod has operated mostly in EDA (pedestal n*>1) • Recent work has extended C-Mod H-modes to lower edge collisionality • Strong shaping (Hubbard, Terry) • High field (~8T) • Unfavorable grad-B drift • Should further explore these regimes, and also attempt to lower n* with cryopumping Adapted from Oyama, PPFC 48 (2006) A171 Slide 2 of 3

  3. Goal: Further explore low collisionality H-modes using all available techniques • Principal questions to explore: • Critical gradient behavior persistent at low n*? • Pedestal width enhancement? • Can we make a more complete mapping of ELM/ELM-free/EDA regimes in edge parameter phase-space? • Do we reproduce Type I ELM size and frequency scale seen on other machines? • Will we “decouple” Ti from Te at sufficiently low collisionality? • Experiments required • Existing MPs: Complete pedestal scaling studies at 8T; more run time in reversed-BT LSN • New proposals • Forward-BT USN (~1/2 day) • Explore shape dependence of our baseline ELMy shape (~1 day) • Use cryopump to reduce density in standard shape, IP (1 day following cryopump development) • Considerable overlap here with other ideas (Hubbard, Terry, et al.) • All new runs should utilize all available edge diagnostics (n, Te, Ti,vq, Er) Slide 3 of 3

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