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

Abstract

Neo-classical tokamak plasma theory predicts poloidal rotation driven by the temperature gradient of order ~ few km/s. In conventional aspect ratio tokamak plasmas, e.g. on JET and DIII-D, poloidal velocities considerably in excess of the neo-classical values have been measured, particularly in the presence of internal transport barriers (ITBs), by means of charge-exchange recombination spectroscopy (CXRS) on the fully ionised C 6+ 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 magnetised plasma and the energy dependence of the charge-exchange excitation process. In present day spherical tokamak (ST) 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 C 6+ impurities are available from high-resolution Doppler CXRS measurements, where the appropriate corrections for the pseudo-velocities are made. Comparison of the measured C 6+ 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 [1, 2], which calculates the full neo-classical transport matrix for bulk ions and a single impurity species for a strongly rotating plasma, as well as a simpler neo-classical theory [3] for an impure plasma. Initial results for both Land H-mode plasmas show that, within the measurement uncertainties, the measured poloidal rotation of the core plasma is consistent with the neo-classical predictions.

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Experimental determination of the correlation properties of plasma turbulence using 2D BES systems

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