Relaxation phenomena and on axis q limit in a reversed field pinch

For sustained discharges at high values of the pinch parameter Θ large disturbances appear on the plasma current, loop voltage, and toroidal magnetic field signals. An increase of both plasma resistance and magnetohydrodynamic (MHD) activity measured at the edge is observed. The onset of such phenomena is found at values of Θ>1.5 and the amplitude of the disturbances increases with Θ. The oscillations are interpreted as the effect of major current redistributions occurring periodically in the central region of the plasma. Relating experimental field profiles and Θ values, the effect of the current redistribution is the recovery of the on axis q value when a lower limit is exceeded. From the experimental measurements such a lower limit has been found to be approximately q(0)≥2a/(3R).