Edge-driven rotating magnetic field current drive of field-reversed configurations

Abstract

Field-reversed configurations (FRCs) are created and sustained using a rotating magnetic field (RMF) in the Translation Confinement and Sustainment experiment. Normally this experiment is operated in a manner where the RMF only partially penetrates the plasma column. This method of operation may have significant advantages in producing less disturbances to the bulk of the FRC, but requires driving an overall radially inward flow to maintain Eθ(r)=0 everywhere (through the VrBz term in the generalized Ohm’s law). However, some RMF penetration is still required at the field null R, where Bz=0. For some experimental conditions it appears that the RMF does not even penetrate as far as the null, raising the question as to how Eθ(r=R) can be maintained at zero despite a finite η⊥jθ(r=R). Numerical simulations with a resistivity profile that is sharply peaked near the plasma edge yield similar profiles, and provide insight into this physical process. An inner magnetic structure forms, which rotates at a much lower frequency than the RMF. A tearing and reconnection process produces a torque transfer from the outer RMF to the inner structure, allowing it to act as an RMF downshifted to a lower frequency, and thus provide current drive to the inner region of the FRC. This mode of RMF current drive is being called “edge-driven mode.”