Maintenance of a stable current profile in a reversed field pinch

Abstract

Externally produced magnetic fields could be used to maintain the reversed field pinch (RFP) plasma confinement configuration in a stable current, j‖∕B, profile. This would be accomplished by the creation of stochastic magnetic field regions in the vicinity of rational surfaces on which the safety factor q is the reciprocal of an integer N. External coils simultaneously excite the poloidal mode number unity and toroidal mode number N and N−1 helical fields from N=Nmax on down in a peristaltic manner, where Nmax is given by the maximal value of q in the RFP plasma. The time scale for turning off and on the N and N−1 pairs of coils is given by τw≡(μo∕η)w2, where w=a∕2Nmax is a typical island half-width required for island overlap, which implies local magnetic stochasticity, and a is the minor radius of the plasma. The total time scale for a complete current spreading cycle is Nmaxτw=(μ0∕η)a2∕4Nmax, which is a factor of Nmax≈10 faster than the time scale for the overall relaxation of the j‖∕B profile.