Comparison of measured poloidal rotation in MAST spherical tokamak plasmas with neo-classical predictions

Neo-classical tokamak plasma theory predicts poloidal rotation driven by the temperature gradient of a few km s(-1). In conventional aspect-ratio tokamak plasmas, e.g. on JET and DIII-D, apparent poloidal velocities considerably in excess of the neo-classical values have been measured, particularly in the presence of internal transport barriers, by means of charge-exchange recombination spectroscopy (CXRS) on the fully ionized C6+ impurity ions. Comparison between such measurements and theoretical predictions requires careful corrections to be made for apparent 'pseudo' velocities, which can arise from the finite lifetime of the excited atoms in the magnetized plasma and the energy dependence of the charge-exchange excitation process. In present day spherical tokamak plasmas this correction is an order of magnitude smaller than on large conventional tokamaks, which operate at higher temperature and magnetic field, hence reducing any associated systematic uncertainties. On MAST measurements of toroidal and poloidal flows of the C6+ impurities are available from high-resolution Doppler CXRS measurements, including appropriate corrections for the pseudo-velocities. Comparison of the measured C6+ velocities with neo-classical theory requires calculation of the impurity flow, which differs from that of the bulk ions due to the respective diamagnetic contributions for each species and inter-species friction forces. Comparisons are made with the predictions of a recent neo-classical theory (Newton 2007 Collisional transport in a low collisionality plasma with strong rotation PhD Thesis University of Bristol, Newton and Helander 2006 Phys. Plasmas 13 102505), which calculates the full neo-classical transport matrix for bulk ions and a single impurity species for a strongly rotating plasma, as well as those of