Toroidal plasma rotation in the TCV tokamak
The first toroidal rotation measurements in TCV ohmic L-mode plasmas with no external momentum injection are presented. The toroidal velocity profile of the fully stripped carbon species is measured by active Charge eXchange Recombination Spectroscopy with a temporal resolution of typically 90 ms and a spatial resolution of 2.5 cm, about 1/10 of the plasma radius. The observed carbon velocity is of the order of the deuterium diamagnetic drift velocity and up to 1/5 of the deuterium thermal velocity. It is directed opposite to plasma current in the electron diamagnetic toroidal drift direction. The profile reverses when reversing the plasma current. The angular velocity profile is flat, or hollow, inside the sawtooth inversion radius and decreases quasi linearly towards the plasma edge. By vertically shifting the plasma magnetic axis within the TCV vessel the plasma edge velocity profile was measured with high resolution. Such experiments confirm that, close to the limiter, the stationary rotation velocity is close to zero or somewhat positive (co-current directed). This suggests that the angular momentum is not driven from the plasma edge. The maximum carbon velocity scales as v(phi,Max) [km s(-1)] = -12.5T(i)/I-p [eV/kA] for a significant range of densities and values of the edge safety factor. Comparison with neoclassical predictions show that the TCV plasma rotation is mainly driven by radial electric fields, with a negligible contribution from the toroidal electric fields. The neoclassical theory of small toroidal rotation quantitatively and qualitatively disagrees with the experimental observation. An alternative empirical equation for the angular momentum flux, able to reproduce the measured stationary profile outside the inversion radius, is proposed.