Bulk plasma rotation in the TCV tokamak in the absence of external momentum input

Abstract

Carbon ion velocity profiles are measured in TCV with a charge exchange diagnostic using a negligibly perturbing diagnostic neutral beam. These ‘intrinsic’ rotation profiles are measured up to the plasma edge in the toroidal and poloidal directions for both limited and diverted plasma configurations in Ohmic plasmas and in the presence of strong second harmonic electron cyclotron heating (ECH). Absolute toroidal velocities are shown to scale with peak ion temperature and inversely with plasma current. The plasma edge rotation is always small in limited configurations but evolves smoothly with the core density for diverted configurations. A strong intrinsic rotation builds up in the plasma core in the counter-current direction for limited configurations but is observed in the co-current direction for diverted plasmas. Unexpectedly, above a given density threshold, the rotation profile reverses to the co-current direction for limited configurations (and surprisingly, in the counter-current direction for diverted configurations). This threshold density is found to depend on plasma current, the presence of ECH and the magnetic topology. Poloidal velocity measurements are used to deduce the radial electric field change across the transition. A strong dependence of the rotation profile on plasma triangularity is reported and possible physics models for these observations are discussed. The origin of the momentum drive, its reversal and its magnitude are not yet clearly understood even for these relatively ‘simple’ experimental configurations.