Experimental study of the relaxation cycle of a decaying spheromak in an external magnetic field

Abstract

For high electron temperature plasma discharges in the S-1 spheromak device [Plasma Physics and Controlled Nuclear Fusion Research, 1984 (IAEA, Vienna, 1985), Vol. 2, p. 535], ‘‘sawtooth’’- like oscillations appear on signals of magnetic field, flux, q value, and electron temperature. Based on the internal magnetic field profiles measured by magnetic probe scans, the mechanisms and causes of these oscillations are revealed. The cycle of one oscillation consists of a toroidal current peaking phase and a subsequent relaxation phase. In the peaking phase, resistive current decay at the edge causes the spheromak to deviate from the initial minimum-energy Taylor state. The deviation was revealed experimentally by the preferential decay of toroidal flux over poloidal flux. A simple calculation shows that a peaking of the electron temperature profile is the most probable cause for the preferential decay of toroidal flux over poloidal flux. During the peaking phase, q decreases so low as to make the configuration unstable to low-n magnetohydrodynamic (MHD) modes (mainly n=2 mode). In the relaxation phase, these modes invoke current redistribution (relaxation), restoring the Taylor state. A significant finding in the relaxation phase is that a reversed toroidal field similar to that of the reversed-field pinch (RFP) configuration is sometimes measured at the edge of the plasma for a brief period. The disappearance or resistive decay of the reversed toroidal flux is attributed to a flux conversion through the magnetic reconnections caused by the low-n modes.