Theoretical and experimental studies of the reversed-field pinch

Recent studies of the reversed-field pinch (RFP) are presented. Stability theory has been extended to include both growth rate and marginal analyses of the tearing mode. The localized resistive interchange instability (g-mode) has been investigated numerically in cylindrical geometry. It is found that there are tearing-mode stable configurations with β approximately <17%, but the stabilizing effects of ion viscosity and finite Larmor radius on the g-mode are less effective than previously found in a slab model and these modes are hard to suppress at high-β. A simple zero-dimensional code which simulates the setting up of RFP distributions by self-reversal is described in which Ohmic heating is balanced against radiation and turbulent losses using experimentally based values to describe these effects. Agreement is found with data from Zeta and the code is used to predict the performance of a new RFP experiment. Experimental studies of the mechanism of self-reversal on HBTXI for compressed and weakly compressed pinches are discussed and measurements presented in which the risetime is longer than the characteristic time for relaxation. In compressed pinches self-reversal is produced by a single large-amplitude m=1 kink instability, whereas in weakly compressed discharges it is due to the cumulative effect of a number of small-amplitude m=1 modes which grow successively. The magnitude of the observed change in Bsub(phi) is explained on this basis. A universal F-theta curve and sustained reversal are observed. The parameters of the next generation of RFP devices are discussed. Reactor studies highlight some problems peculiar to the RFP, but establish it as a viable alternative to the tokamak, with advantages if Ohmic ignition and high-β can be realized. (author)