(EX/P6-4) Comparison of Small ELM Characteristics and Regimes in Alcator C-Mod, MAST, and NSTX

R. Maingi1), A.E. Hubbard2), H. Meyer3), J.W. Hughes2), A. Kirk3), R. Maqueda4), J.L. Terry2), Alcator C-Mod Team, MAST Team, NSTX Team
1) Oak Ridge National Laboratory, Oak Ridge, TN, United States of America
2) Massachusetts Institute of Technology, Cambridge, MA, United States of America
3) UKAEA Fusion, Culham, United Kingdom
4) Nova Photonics, Princeton, NJ, United States of America

Abstract.  We report on a set of ITPA-endorsed experiments among the Alcator C-Mod, MAST and NSTX devices to compare the characteristics of small edge-localized modes (ELMs) and the conditions in which they can be obtained, with the goal of assessing their extrapolability to ITER. The urgency for development of small and no ELM scenarios has increased with the recent revision of the allowable ELM size in ITER to 1 MJ, representing about 0.3% of the 350 MJ plasma stored energy. In Alcator C-Mod, the Enhanced Dα (EDA) regime has been shown to have individual small ELMs at sufficiently high pedestal temperature and/or pedestal b. In MAST small ELMs have been observed in specific discharge scenarios and shapes. Finally in NSTX a small ELM regime, termed Type V ELMs, has been shown to have a wide operating window with unique ELM structure. A second small ELM regime was also found in NSTX. One straightforward conclusion from these studies is that small ELMs can indeed have different toroidal mode structure and operational windows, even within a single device. Thus there may be multiple scenarios by which small ELMs could be achieved naturally in ITER. A common observation from all of the devices is an apparent bped threshold for small ELM access, although the threshold value varied widely between the devices. Stability analysis of these ELMs is complicated by the fact that the profile relaxation following small ELMs is quite subtle. The status of simulations of these results with M3D will be presented.

Sponsored in part by U.S. Dept. of Energy Contracts DE-AC05-00OR22725, DE-FC02-99ER54512, DE-FG02-04ER54767, and DE-AC02-76CH03073, and the UK Engineering and Physical Sciences Research Council.

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