Observations and analysis of magnetic fluctuations in the Maryland centrifugal experiment

Abstract

Initial results from magnetic probes on the Maryland Centrifugal eXperiment (MCX) [R. F. Ellis et al., Phys. Plasmas 8, 2057 (2001)] provide details of the propagation and azimuthal mode structure of magnetic fluctuations in the edge region. Magnetic coils placed azimuthally along the edge measure changes in the axial magnetic field during the time history of the plasma discharge. The eight evenly spaced coils can resolve azimuthal modes up to m=3. The plasma rotates azimuthally in MCX due to an applied radial electric field. Using a variety of different analysis of the data, it is inferred that the magnetic fluctuations are dominantly convected by the plasma rotation for several rotation periods before significant decorrelation. These findings help to identify the modes at the edge and indicate that there are a few low mode numbers that are dominant during the discharge. Also, the speed of rotation of the modes is found to change dramatically from the High Rotation (HR) state to a low rotation ordinary (O) state, consistent with a corresponding change in the plasma load voltage. The fluctuation spectrum becomes dominated by a single mode after the transition.