Observation of ion cyclotron range of frequencies mode conversion plasma flow drive on Alcator C-Mod

Abstract

At modest H3e levels (n3He/ne∼8%–12%), in relatively low density D(H3e) plasmas, n¯e≤1.3×1020 m−3, heated with 50 MHz rf power at Bt0∼5.1 T, strong (up to 90 km/s) toroidal rotation (Vϕ) in the cocurrent direction has been observed by high-resolution x-ray spectroscopy on Alcator C-Mod. The change in central Vϕ scales with the applied rf power (≤30 km s−1 MW−1), and is generally at least a factor of 2 higher than the empirically determined intrinsic plasma rotation scaling. The rotation in the inner plasma (r/a≤0.3) responds to the rf power more quickly than that of the outer region (r/a≥0.7), and the rotation profile is broadly peaked for r/a≤0.5. Localized poloidal rotation (0.3≤r/a≤0.6) in the ion diamagnetic drift direction (∼2 km/s at 3 MW) is also observed, and similarly increases with rf power. Changing the toroidal phase of the antenna does not affect the rotation direction, and it only weakly affects the rotation magnitude. The mode converted ion cyclotron wave (MC ICW) has been detected by a phase contrast imaging system and the MC process is confirmed by two-dimensional full wave TORIC simulations. The simulations also show that the MC ICW is strongly damped on H3e ions in the vicinity of the MC layer, approximately on the same flux surfaces where the rf driven flow is observed. The flow shear in our experiment is marginally sufficient for plasma confinement enhancement based on the comparison of the E×B shearing rate and gyrokinetic linear stability analysis.