The Lundquist number scaling of nonlinear MHD fluctuations and transition from quasi-continuous to discrete reconnection activity in MST RFP plasmas

Nonlinear MHD fluctuations appear in both natural and magnetic confinement settings, such as the solar wind, self-organization dynamics in the RFP and spheromak, and current disruptions in tokamak plasmas. Here we describe parameter scaling experiments oriented toward nonlinear MHD dynamics in RFP plasmas. Experimental data have been gathered spanning a wide range of parameter space characterized by Lundquist number, S ∼ 10⁴ −10⁷, and density, n_e/n_G, where n_G is the empirical density limit. A new programmable power supply allows low-current, low-S operation, which overlaps with parameters available in numerical modeling. Experimental S scalings of magnetic fluctuation amplitude agree well with those from the nonlinear MHD codes DEBS and NIMROD. A transition from quasi-continuous activity to bursty relaxation having discrete sawtooth events is observed in going from low to high S, with a threshold at around S ∼ 10⁵. The spectral properties of the magnetic fluctuations change at this transition, including a reduction in fluctuation phase velocity that suggests plasma flow and/or flow profile changes. Momentum transport and flattening of the flow profile are known features associated with sawtooth relaxation in RFP plasmas.