New Observations Concerning the Origin and Consequences of MHD Activity in the MST Reversed Field Pinch

Reversed Field Pinch (RFP) plasmas exhibit a broad spectrum of MHD activity. The instability underlying much of this activity in the standard RFP discharge has been identified as resistive tearing and is well modelled by nonlinear, resistive MHD. In recent years two additional types of discharge have been produced: those with reduced tearing activity leading to improved confinement and those with one large mode termed Quasi-Single Helicity (QSH). Our understanding of MHD activity and its consequences in all three plasma types is incomplete. In this paper, we present new data concerning (1) the origin of m=0 modes in standard plasmas, (2) the MHD dynamo in QSH plasmas, (3) the effect of MHD modes on plasma rotation, and (4) the achievement of a nearly dynamo-free RFP plasma.