A method for sustaining a current in a neutral-beam-driven field-reversed mirror is presented. It has been shown that steady-state currents may not be driven in symmetric field-reversed configurations without stationary impurities. The plasma electrons are accelerated by collisional interaction with the beam-produced ion current, causing the net current to vanish asymptotically in time. It is shown that the current cancellation can be avoided by weakly breaking the toroidal symmetry of the device, which allows angular momentum transfer between the electrons and the external coil structures without having plasma in contact with material surfaces. The momentum transfer is manifested as an additional force on the electrons, preventing them from accelerating to the ion velocity and cancelling the current. The detailed mechanism for transferring momentum is magnetic pumping (synonymous with parallel viscosity, non-adiabatic pressure effects). The analysis is applied to existing and planned mirror experiments indicating viability of the effect for reasonable plasma and beam parameters.
Accretion theory of `spontaneous' rotation in toroidal plasmas