Effects of current and pressure profiles on resistive MHD modes in reversed field pinches

Abstract

Resistive MHD stability studies of reversed field pinches (RFPs) for various mu profiles (where mu = mu 0J.B/B2) and pressure profiles have been done. For the first time, a system that can simulate both the experimental hollow mu profiles and the locally flattened mu profiles has been set up, thus enabling extensive stability studies of equilibrium profiles. Broadening the mu profile is beneficial to stabilizing the internal mode. A mu profile locally flattened around the reversal surface is found to cause a deterioration of the internal mode and the m=0 mode stability. Also, the corresponding spectra of unstable modes are expanded. This may be associated with the increasing magnetic fluctuations. When a mu profile is hollow near the magnetic axis, the spectrum shrinks to low kz, values and the field reversal is deepened. Whether this hollow mu profile causes a deterioration of the stability in RFP depends on the depth of the hollowness and on the corresponding pinch parameter. On the other hand, unstable modes in the RFP are found to be very sensitive to the pressure gradient at the rational surface. Even for the typical beta p=10% case, the shaping of the pressure profile dramatically changes the stability of both the plasma core and the plasma boundary regions. Furthermore, the poloidal beta, beta p, is found to be sensitive to the modification of the current profile. The resulting stability in the RFP when changing the current profile is actually due to the combined effect of changing both beta p and the mu profile. This shows the limitation of the "force-free" assumption, which is widely used in RFP simulations