Electromagnetic energy transport by tearing fluctuations during magnetic relaxation in a self-organized reversed-field pinch plasma

A radial Poynting flux due to tearing mode fluctuations during magnetic relaxation is measured in the Madison Symmetric Torus (MST), a reversed-field pinch plasma at the Wisconsin Plasma Physics Laboratory (WiPPL). We find that during sawtooth crash relaxation events, this flux corresponds to transient power levels larger than the input power, comparable to the global equilibrium magnetic energy transient loss rate, and sufficient to drive the fluctuation-induced dynamo EMF supporting the plasma's magnetic self-organization. The edge radial profile of the time-average flux is observed to reach a maximum at the magnetic reversal surface, where it corresponds to approximately 65% of the input power, while at the extreme edge it corresponds to approximately 20% of the input. A simple Poynting's theorem model for an incompressible, resistive MHD plasma with resistive boundary is developed, predicting that the fluctuation-induced Poynting flux out of the plasma corresponds approximately to the power lost from the equilibrium magnetic field due to the dynamo EMF. Experimental probe measurements of this flux are roughly as predicted by the model upon substitution of time-resolved equilibrium measurement data.