Achieving a steady state field-reversed configuration by means of two counter-rotating magnetic fields

Abstract

A field-reversed configuration may be sustained by a rotating magnetic field (RMF) which entrains the electron fluid and maintains the plasma current (the (-) RMF). This current would eventually decay as the ion fluid is accelerated through momentum transfer collisions with the electron fluid. A true steady state operation may be achieved by applying a second counter-RMF (the (+) RMF) which imparts angular momentum to the ion fluid, thereby overcoming the collisional torque. We present a theoretical model of RMF current drive demonstrating the physical basis of the scheme, derive the conditions required for successful operation and present simulation results. Both RMFs may penetrate much farther than the classical skin depth due to nonlinear effects. The required external RMF magnitudes are of the same order for both RMFs and follow the same scaling law. The (+) RMF magnitude required for penetration is much larger than that required to sustain the steady state, providing two critical values for (the ratio of the electron cyclotron frequency in the (+) RMF and the electron ion collision frequency), which scale linearly with (the ratio of the plasma radius and the classical skin depth for the frequency of the (+) RMF).