Studies of a reversed field pinch in a poloidal divertor configuration

An attempt has been made to form a reversed field pinch (RFP) in a poloidal divertor configuration which positions the plasma far from a conducting wall. In this configuration, the plasma is localized within a magnetic separatrix formed by a combination of toroidal currents in the plasma and four internal aluminium rings. Plasmas were formed with a plasma current of ~135 kA, toroidal field reversal lasting ~1 ms, line averaged density of ~( l -2) X 10 cm" and central electron temperature of ~50 eV, but a large asymmetry in the magnetic field (6B/B ~ 40%) set in at about the time when the toroidal field reversed at the wall. This behaviour might be expected on the basis of MHD stability analysis of a cylindrical plasma bounded by a large vacuum region and a distant conducting wall. The symmetric equilibrium before the asymmetry develops and the asymmetry itself are described.