Comparison of confinement in resistive-shell reversed-field pinch devices with two different magnetic shell penetration times

A loop voltage is required to sustain the reversed-field pinch (RFP) equilibrium. The configuration is characterized by redistribution of magnetic helicity but with the condition that the total helicity is maintained constant. The magnetic field shell penetration time, τs, has a critical role in the stability and performance of the RFP. Confinement in the EXTRAP device has been studied with two values of τs, first (EXTRAP-T2) with taus of the order of the typical relaxation cycle timescale and then (EXTRAP-T2R) with τs much longer than the relaxation cycle timescale, but still much shorter than the pulse length. Plasma parameters show significant improvements in confinement in EXTRAP-T2R. The typical loop voltage required to sustain comparable electron poloidal beta values is a factor of 3 lower in the EXTRAP-T2R device. The improvement is attributed to reduced magnetic turbulence