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(EX1/4) H-Mode Regimes and Observations of Central Toroidal Rotation in ALCATOR C-Mod

   
M. Greenwald , J. Rice , R. L. Boivin , P. Bonoli , R. Budny ², C. Chang ¹, D. Ernst ², C. Fiore , J. Goetz , R. Granetz , A. Hubbard , I. Hutchinson , J. Irby , B. LaBombard , B. Lipschultz , E. Marmar , D. Mossessian , M. Porkolab , W. Rowan ⁴, J. A. Snipes , G. Schilling ², Y. Takase ⁵, J. Terry , S. Wolfe , J. Weaver ³, B. Welch ³, S. Wukitch 
 
MIT - Plasma Science & Fusion Center
¹ New York University
² Princeton Plasma Physics Laboratory
³ University of Maryland
⁴ University of Texas at Austin
⁵ University of Tokyo

Abstract
The Enhanced $\rm D_\alpha$ or EDA H-mode regime in Alcator C-Mod  has been investigated and compared in detail to ELM-free plasmas. (In this paper, ELM-free will refer to discharges with no type I ELMs  and with no sign of EDA, though technically, most EDA plasmas are ELM-free as well.) EDA discharges have only slightly lower energy confinement  than comparable ELM-free ones, but show markedly reduced impurity confinement. Thus EDA discharges do not accumulate impurities and typically have a lower fraction of radiated power. EDA plasmas are seen to be more likely at low plasma current (q $>$ 3.7 - 4), for moderate plasma shaping, (0.35 - 0.55), and for high neutral pressures. No obvious trends were observed with input power or pressure ($\beta$). In both H-mode  regimes, and in ICRF heated L-modes, central impurity toroidal rotation has been deduced, from the Doppler shifts of argon x-ray lines. Rotation velocities up to $1.3 \times 10^5$ m/s in the co-current direction have been observed in H-mode discharges that had no direct momentum input. There is a strong correlation between the increase in the central impurity rotation velocity and the increase in the plasma stored energy, induced by ICRF heating. In otherwise similar discharges with the same stored energy increase, plasmas with lower current rotate faster. The ion pressure gradient is an unimportant contributor to the central impurity rotation and the presence of a substantial core radial electric field is inferred during the ICRF pulse. An inward shift of ions induced by ICRF waves could give rise to a non-ambipolar electric field in the plasma core. Comparisons with a neo-classical ion orbit shift model show good agreement with the observations, both in magnitude, and in the scaling with plasma current.

   

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