Reversed-field pinch research in the United Kingdom

Abstract

The foundations of present-day RFP research are to be found in the early results from ZETA in 1967, in which a period of improved stability was observed in discharges with a spontaneously generated reversed field, during which the magnetic field configuration was in accord with ideal-MHD stability requirements. Theoretical work at Culham subsequently established that the reversed-field configuration is a minimum magnetic energy state to which a plasma has a strong propensity to relax naturally. – HBTX1, designed to study fastprogrammed RFPs in a quartz torus, became operational during 1972; spontaneous field reversal was again demonstrated. By operating with current risetimes shorter or longer than the natural relaxation time, both reversal by strong helical instability and relaxation to reversed field more gently along a minimum energy trajectory in F-θ space were demonstrated. However, although much important information was obtained from HBTX1, including checking the predictions of relaxation and resistive instability theory, high temperatures could not be sustained,and ZETA-like quiescence was not found; this was attributed to the use of an insulating vessel. – A new assembly, HBTX1A, with a stainless-steel liner, was commissioned in 1981. By using a steady vertical field to assist in providing equilibrium, plasma lifetimes of up to 15 ms were observed with electron temperatures of up to 350 eV (at 200 kA) at densities of (1–2) × 1019 m−3. Magnetic field fluctuations with B̃/B ≥ 1–2%, seen throughout the discharge, are dominated by poloidal m = 0 and m = 1 global modes of resistive MHD origin, and localized activity in the core at higher frequencies. The m = 1 modes can contribute to anomalous energy transport, and to reversed-field penetration. The value of βθ≈ 10% varies relatively little with conditions in HBTX1 A, which is consistent with theoretical predictions of transport due to g-modes; if βθ remains constant at about 10% as the current and machine size are increased reactor conditions will be reached.