Measuring fluctuation-driven current and electromagnetic energy transport in a reversed-field pinch plasma*

In inductively-driven reversed field pinch plasmas, the equilibrium magnetic configuration is determined by self-organization processes resulting in a toroidal magnetic field that is peaked in the core and decreases monotonically with minor radius. The edge magnetic field gains a component directed opposite the core toroidal field. The self-organization process is characterized by a ‘sawtooth’ cycle in which tearing modes reconnect and drive a nonlinear energy cascade, exciting broadband and multimodal fluctuations. Coherent fluctuations cause current transport by inducing a ‘dynamo’ EMF 〈v~×B~〉_|| ≈〈E~●B~〉/B²  parallel to the local equilibrium magnetic field, enhancing the poloidal current in the edge. Coherent fluctuations in electric and magnetic fields also result in an outwardly-directed Poynting flux S_f =μ₀^-1∫E~×B~ dA. An insertable probe measures fluctuating electric and magnetic fields on the MST device at the Wisconsin Plasma Physics Lab (WiPPL). The behaviors of the dynamo EMF and S_f are explored for radial locations both inside and outside the magnetic reversal surface. The S_f measurements correspond approximately to the power lost from the equilibrium magnetic field due to the dynamo EMF, as predicted by a simple Poynting’s theorem model for an incompressible, resistive MHD plasma with resistive boundary. Both quantities show significant radial variation, and the dynamics behind this variation are investigated.