H-MODE REGIMES AND OBSERVATIONS OF CENTRAL TOROIDAL ROTATION IN ALCATOR C-MOD

摘要

The Enhanced Da or EDA H-mode regime in Alcator C-Mod has been investigated andrncompared in detail to ELM-free plasmas. (In this paper, ELM-free will refer to discharges with norntype I ELMs and with no sign of EDA, though technically, most EDA plasmas are ELM-free as well.)rnEDA discharges have only slightly lower energy confinement than comparable ELM-free ones, butrnshow markedly reduced impurity confinement. Thus EDA discharges do not accumulate impuritiesrnand typically have a lower fraction of radiated power. EDA plasmas are seen to be more likely at lowrnplasma current (q > 3.7 - 4), for moderate plasma shaping, (0.35 - 0.55), and for high neutral pressures.rnNo obvious trends were observed with input power or pressure (b). In both H-mode regimes, and inrnICRF heated L-modes, central impurity toroidal rotation has been deduced, from the Doppler shifts ofrnargon x-ray lines. Rotation velocities up to 1.3 x 10~5 m/s in the co-current direction have beenrnobserved in H-mode discharges that had no direct momentum input. There is a strong correlationrnbetween the increase in the central impurity rotation velocity and the increase in the plasma storedrnenergy, induced by ICRF heating. In otherwise similar discharges with the same stored energyrnincrease, plasmas with lower current rotate faster. The ion pressure gradient is an unimportantrncontributor to the central impurity rotation and the presence of a substantial core radial electric field isrninferred during the ICRF pulse. An inward shift of ions induced by ICRF waves could give rise to arnnon-ambipolar electric field in the plasma core. Comparisons with a neo-classical ion orbit shift modelrnshow good agreement with the observations, both in magnitude, and in the scaling with plasmarncurrent.