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

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 C-Mod Ideas Forum: 13—15 Dec 2006 J.W. Hughes, “Continued exploration of . . .” (Transport TF) Slide 2 of 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) C-Mod Ideas Forum: 13—15 Dec 2006 J.W. Hughes, “Continued exploration of . . .” (Transport TF) Slide 3 of 3