OVERRELAXATION PHENOMENA DURING THE FORMATION OF REVERSED-FIELD PINCHPLASMAS

Experiments on the Extrap T1 reversed-field pinch (RFP) [Phys. Scr. 49 , 224 (1994)] have shown that the formation of the RFP configuration i s quite sensitive to the relative programming of the toroidal field an d Ohmic heating circuits. In this paper, new measurements of the evolu tion of the current density profile and of the spectral structure of t he fluctuations during the setup phase of RFP plasmas in the T1 experi ment are presented. These measurements improve the understanding of th e role of different spectral components in the dynamics of RFP formati on. Under unfavorable (slow) setup conditions, comparatively high ener gy is accumulated in m = 1 internal kinks prior to reversal of the edg e toroidal field. At reversal, nonlinearly driven m = 0 modes trigger a rapid broadening of the m = 1 spectrum. This behavior is associated with a violent suppression of the current density in the core, leading to an overrelaxation of the discharge involving a hollowing of the pa rallel current density profile. The setup conditions are found to affe ct the volt-second consumption and plasma/wall interaction during RFP formation, as well as the flat-top discharge performance. (C) 1995 Ame rican Institute of Physics.