Formation mechanisms of the inversed current in the edge region of RFP plasma

The edge toroidal reversed current outside the field reversal surface manifestly plays some important roles in reversal field pinches (RFPs). The crucial roles of the edge reversed toroidal current has included the implication of the deep field reversal in the recently attained improved confinement modes in some RFPs. We have concentrated our attention on the formation mechanism of the reversed edge toroidal current in the STP-3M reversed field pinch. Measurements of fast electron current and floating potential have revealed minutely that the reversed edge toroidal current is driven by fluctuational transport of superthermal electrons produced in the core region. Time evolution of the floating potential, measurement of the space potential profile, has shown the radially outerward electric field, an indication of the preferential loss of electrons from the core region. The dominance of the core resonant modes in the toroidal mode power spectrum of the edge magnetic fluctuations indicates the stochastic magnetic field line in the core region. This is to the evidence that the origin of the edge fast electrons is the stochastic diffusion of fast electrons produced in the core region. (author)