Reversed field pinches

Abstract

The present status of RFP research is reviewed with emphasis on recent experimental developments. The basic properties of the RFP are summarised in section 2 including equilibrium and relaxed states, self-reversal and stability. The remainder of the paper deals with experimental results. There are five intermediate sized machines operating with the minor radii of the metal bellows liner in the range 9–26 cm, peak currents of a few hundred kA reached in between 0.1 and 4 ms with pulse lengths of up to more than 10 ms. The field configuration is set up using self-reversal usually assisted by slow B φ control. The temperatures are typically a few hundred eV (maximum ∼ 600 eV on TPE-1R(M) in Japan) and the density is typically 10 13–10 14 cm −3 with maximum β θ ≳10%. It is found that some plasma properties depend on the value of I/ N ( I is the current and N the line density) with a clear high-density limit due to radiation. The electron temperature increases with current; much of the data fits a dependeence T ∝ I n where 0.5 < n < 1 and the most consistent data indicates n ≲ 1. The measured energy confinement time, τ E, is composed with classical predictions of tokamak scaling laws. The value of τ E is less than for classical ion crossfield conduction by ∼ 1 10 – 1 50 ; The precise factor depends on β θ 2 (where β θ is the measured value) and on the radial distribution. The data do not fit tokamak scaling laws which are compared with possible pinch scaling. In the final section, some of the main problems of reversed field pinch research will be highlighted with an indication of future trends in this work.