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(EX/P5-04) H-mode Edge Stability of Alcator C-Mod Plasmas

D.A. Mossessian¹⁾, A. Hubbard¹⁾, J.W. Hughes¹⁾, M. Greenwald¹⁾, B. LaBombard¹⁾, J.A. Snipes¹⁾, S. Wolfe¹⁾, P. Snyder²⁾, H. Wilson³⁾, X. Xu⁴⁾, W. Nevins^4)
 
1) Massachusetts Institute of Technology, Plasma Science and Fusion Center, Cambridge, USA
²⁾ General Atomics, San Diego, USA
³⁾EURATOM/UKAEA Fusion Association, Culham Science Centre, UK
⁴⁾Lawrence Livermore National Laboratory, Livermore, USA

Abstract.  For steady state H-mode operation, a relaxation mechanism is required to limit build-up of the edge gradient and impurity content. C-Mod sees two such mechanisms - EDA and grassy ELMs, but not large type I ELMs. In EDA the edge relaxation is provided by an edge localized quasicoherent electromagnetic mode that exists at moderate pedestal temperature T< 400 eV, high pedestal density and high edge safety factor, q>3.5 and does not limit the build up of the edge pressure gradient. The mode is not observed in the ideal MHD stability analysis, but is recorded in the nonlinear real geometry fluctuations modeling based on fluid equations and is thus tentatively identified as a resistive ballooning mode. At high edge pressure gradients and temperatures the mode is replaced by broadband fluctuations (f < 50 kHz) and small irregular ELMs are observed. Based on ideal MHD calculations that include the effects of edge bootstrap current, these ELMs are identified as medium n (10 < n < 50) coupled peeling/ballooning modes. The stability thresholds, its dependence on the plasma shape and the modes structure are studied experimentally and with the linear MHD stability code ELITE.

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