Plasma Phys. Control. Fusion 37 (1995) A30SA311. Printed in the UK Plasma rotation, dynamo, and nonlinear coupling in the reversed field pinch S C Prager, A F ALmagri, M Cekic, J Chapman,D J Den Hartog, G Fiksel, C Hegna, H Ji and J S Sarff Department of Physics, University of Wisconsin, Madison, WI 53706 USA J R Drake, S Mazur, P Nordlund and HE Saetherblom Division of Fusion Plasma Physics (Association EUROTAM-NFR), Alfven Laboratory, Royal Institute of Technology, S-100 44 Stockholm Abstract. Two important effects of MHD fluctuations in the RFP and tokamak are current generation (the dynamo effect) and mode locking. In the T I and MST RFP experiments new results reveal the mode dynamics underlying these phenomena. In T1 the effect of specific magnetic Fourier modes on the current density profile is evident. In MST, the MHD dynamo term (6v x 6B) is measured in the plasma edge, and found to account for the time dependence of the edge current throughout a sawtooth cycle. As edge resistivity is increased in T1 the fluctuation amplitude increases to maintain the dynamo-driven current, as expected from MHD computation. The modes responsible for the dynamo often lock to the local magnetic field error at the vertical cut in MST. The plasma rotation velocity has been measured with a fast Doppler spectrometer to a time resolution of 1 ps. The plasma rotation and mode phase velocity are remarkably wekorrelated, with both slowing, in the presence of an impulsive field error, in a 100 ps timescale. 1. Introduction In the reversed field pinch (RFP), magnetic fluctuations have enonnous effect on plasma behavior. In this paper we focus upon two effects, both likely understood from MHD theory - the influence of magnetic fluctuations on the current density profile and the effect on plasma rotation. Both of these phenomena are of importance in other configurations, such as the tokamak. However, in the RFP these effects are large, so that the underlying physics mechanisms can be effectivelystudied. We report new results from the MST and T1 reversed field pinch experiments. MST, at the University of Wisconsin, is a large plasma (a = 0.5 m, R = 1.5 m) with moderate plasma current (I = 0.5 MA), modest current density and aspect ratio of 3. T1 at the Royal Institute of Technology in Stockholmis a small RFP (a = 6 cm, R = 0.5 m, I = 40 - 100 kA) with high current density (10 "2 ) and aspect ratio of 8.8. Observation of consistent effects in both devices supports the generality of the MHD interpretation of the experimental results. The influence of magnetic fluctuations on the current density profile, or magnetic 01995 American Physical Society A303