Confinement improvement and current profile control in the MST reversed-field pinch

Abstract

Summary form only given, as follows. The reversed field pinch (RFP) is a toroidal magnetic configuration with a much weaker toroidal magnetic field compared to a tokamak. RFP behavior is determined in large part by magnetic fluctuations driven by the gradient in the current density. Since the RFP is a q/sub /spl darr//1 configuration, magnetic fluctuations are large and energy confinement is degraded by the associated transport Recently, energy confinement has been significantly improved in the MST reversed field pinch by applying inductive parallel current drive in the outer region of the plasma. Direct measurement of the current density profile and magnetic field fluctuations, by detection of the Faraday rotation of an injected FIR laser beam, indicates that internal magnetic fluctuations and the associated current fluctuations are reduced while current density profile is altered. As a result of reduced magnetic transport, the electron temperature nearly triples and beta doubles. Fast electron confinement is evidenced by a 100-fold increase in hard x ray radiation suggesting reduced magnetic stochasticity and the formation of good flux surfaces. The energy confinement time increases ten-fold, which is comparable to L- and H-mode scaling values for a tokamak with the same plasma current, density, heating power, size and shape. Tokamak quality confinement achieved in the MST reversed field pinch is at much higher beta and lower toroidal magnetic field. However, the confinement improvement is not sustained since the inductive current profile technique is itself transient. Research is underway to apply RF current drive to the RFP for sustained, improved confinement.