ICRF Mode Conversion Flow Drive on Alcator C-Mod

Full text: ICRF mode conversion flow drive (MCFD) may be a candidate for the external control of plasma rotation in large tokamaks like ITER. Recently, we have carried out a detailed study on MCFD, including its dependence on plasma and RF parameters. These results shed some light on the underlying physics and may help to extrapolate the method to other fusion devices. The observed change in the toroidal rotation (dV) is always in the co-Ip direction. The flow drive efficiency is found to depend strongly on the 3He concentration in D(3He) plasmas, a key parameter separating the ICRF minority heating and mode conversion regimes. This result further supports the key role of mode conversion. The efficiency is also strongly affected by plasma density (∼ 1/ne), i.e., a power and/or momentum per particle dependence. The flow drive efficiency at f = 78 MHz and Bt(0) = 8 T is lower than that at 50 MHz/5.1 T, possibly due to the 1/f dependence of wave momentum at the same RF power. The change of rotation also increases with Ip, indicating momentum confinement dependence. At +90 degree antenna phase (waves in co-Ip direction) and dipole phase (waves symmetrical in both directions), we find that dV is proportional to the RF power up to the maximum available RF power. A central dV ∼ 110 km/s in low density L-mode has been achieved. The rotation at -90 degree antenna phase is usually smaller, and the flow drive effect appears to be saturated (or decreased) at high RF power. The difference vs. antenna phases indicates that possibly two mechanisms are involved in determining the total flow drive torque: one is RF power dependent and generates a torque in the co-Ip direction, and the other is wave momentum dependent, i.e., the torque changes direction vs. antenna phase. Results in H-mode follow the same parametric scaling. However, the observed change of rotation in H-mode has been small because of the much higher density and the unfavorable 1/ne scaling. Work supported by U.S. D.o.E. Cooperative Agreement DE-FC02-99ER54512. (author)