Effects of oscillating poloidal current drive on magnetic relaxation in the Madison Symmetric Torus reversed-field pinch

Magnetic relaxation behavior in reversed-field pinch (RFP) experiments on the Madison Symmetric Torus device is modified by oscillating poloidal current drive (OPCD). We observe that OPCD modulates the nonlinear magnetic fluctuation dynamics of the RFP as it modulates the equilibrium and its linear stability properties. In particular, OPCD can entrain the RFP's nonlinear magnetic relaxation cycle and can therefore modify the frequency of the discrete relaxation events called sawtooth crashes. These crashes, which are intermittent or quasiperiodic in standard RFP plasmas without oscillating voltages, can be entrained by OPCD to become nearly periodic. The entrainment of the RFP sawtooth cycle is investigated by varying OPCD amplitude and frequency, as well as plasma equilibrium magnetic field reversal. Impurity ion (C⁺⁴) heating induced and modulated by OPCD is measured by ion Doppler spectrometry and is ascribed to the modulation of magnetic reconnection activity.