Resistive Wall Mode Studies in a Reversed Field Pinch with Rotating Helical Field

In standard RFP plasmas in STE-2, the magnetic fluctuations are dominated by core resonant tearing modes which grow with the time scale of the resistive wall, and these modes are identified as the resistive wall tearing modes. The growth of these modes are shown to be suppressed by the resonant rotating; helical field (RHF). The response of m=l modes to the RHF has been studied in detail. In RFP plasmas, the RHF has the effect on the rotation of neighboring modes as well as of the resonant mode. In ULQ plasmas, where there is no m=0 resonant surface, only the resonant mode tends to be influenced by the extr:rnal resonant perturbation. The importance of the ra=0 mode in the mode coupling process is discussed. Keywords: reversed field pinch, tearing mode, rotating helical field, mode control, external kink mode