Technology and Physics Implications of Oscillating-Field Current Drive in Reversed-Field Pinches

The property of reversed-field pinches (RFPs) to relax to a near-minimum-energy state is the basis of oscillating-field current drive (OFCD), wherein plasma current is driven by modulating in quadrature the external toroidal and poloidal magnetic fields. Coupled plasma/circuit OFCD simulations of RFPs ranging from present experiments (ZT-P and ZT-40M) to the reactor (TITAN) indicate that the drive frequency and the amplitude of the plasma-current oscillations decrease and the “wall-plug” current-drive efficiency increases with decreased plasma resistance so that minimum frequencies (∼25 Hz) and plasma-current amplitudes (∼1.6%) and maximum efficiencies (∼0.3 A/W) are attained in the reactor regime. Methods for minimizing the reactive powers and for optimizing the current-drive efficiency for OFCD in RFPs have been identified.