Measurement and physical interpretation of the mean motion of turbulent density patterns detected by the BES system on MAST

摘要

The mean motion of turbulent patterns detected by a two-dimensional (2D) beamemission spectroscopy (BES) diagnostic on the Mega Amp Spherical Tokamak (MAST)is determined using a cross-correlation time delay (CCTD) method. Statisticalreliability of the method is studied by means of synthetic data analysis. Theexperimental measurements on MAST indicate that the apparent mean poloidalmotion of the turbulent density patterns in the lab frame arises because thelongest correlation direction of the patterns (parallel to the local backgroundmagnetic fields) is not parallel to the direction of the fastest mean plasmaflows (usually toroidal when strong neutral beam injection is present). Theexperimental measurements are consistent with the mean motion of plasma beingtoroidal. The sum of all other contributions (mean poloidal plasma flow, phasevelocity of the density patterns in the plasma frame, non-linear effects, etc.)to the apparent mean poloidal velocity of the density patterns is found to benegligible. These results hold in all investigated L-mode, H-mode and internaltransport barrier (ITB) discharges. The one exception is a high-poloidal-beta(the ratio of the plasma pressure to the poloidal magnetic field energydensity) discharge, where a large magnetic island exists. In this case BESdetects very little motion. This effect is currently theoretically unexplained.