PLASMA ROTATION, DYNAMO, AND NONLINEAR COUPLING IN THE REVERSED-FIELDPINCH

Two important effects of MHD fluctuations in the RFP and tokamak are c urrent generation (the dynamo effect) and mode locking. In the T1 and MST RFP experiments new results reveal the mode dynamics underlying th ese phenomena. In T1 the effect of specific magnetic Fourier modes on the current density profile is evident. In MST, the MHD dynamo term (d elta v x delta B) is measured in the plasma edge, and found to account for the time dependence of the edge current throughout a sawtooth cyc le. As edge resistivity is increased in T1 the fluctuation amplitude i ncreases to maintain the dynamo-driven current, as expected from MHD c omputation. The modes responsible for the dynamo often lock to the loc al magnetic field error at the vertical cut in MST. The plasma rotatio n velocity has been measured with a fast Doppler spectrometer to a tim e resolution of 1 mu s. The plasma rotation and mode phase velocity ar e remarkably well-correlated, with both slowing, in the presence of an impulsive field error, in a 100 mu s timescale.