Polarized radial magnetic fields and outward plasma fluxes during shallow-reversal discharges in the ZT-40M reversed-field pinch

Abstract

The characteristics of edge-region electromagnetic disturbances and of pulsed radial fluxes of plasma to the liner as well as the detailed interrelationship among these processes have been studied on the ZT-40M reversed-field pinch in its normal, shallow-reversal operating regime. The dominant magnetic disturbances are spiky (pulsewidth ∼5–10 μs) low-amplitude (‖Br/Bθ‖≲10−2) poloidally symmetric radial-field structures intersecting the vacuum wall and precessing toroidally in the anti-Iφ sense. The effect of even slight toroidal-field reversal (‖Bφ(a)‖≊Bθ(a)/10) is to polarize these radial-field spikes preferentially positive (i.e., Br>0) and to increase the speed of the minority (Br<0) while decreasing the speed of the majority (Br>0) spikes. Synchronous with the polarized Br spikes are intense radially outward fluxes of plasma (instantaneously ≳1022 m−2 s−1) leading to recurrent, large amplitude (‖Δn/n≳25%) depletion of the density in the outer quarter of minor radius. The resulting time-averaged global loss-rate per particle is significant (∼103 s−1). The toroidal wavenumbers (kφ≊3 m−1) of the radial-field spikes are sufficiently low to cause radially extensive (Δr∼a/4) magnetic islands which could account for the observed pulsed depletions of density in the same radial zone. The propagation and polarization of the coupled radial field/plasma loss process cannot be explained on the basis of known single-fluid plasma models.