Measuring the Poynting flux produced by coherent field fluctuations in a reversed-field pinch plasma

In a reversed-field pinch driven by a toroidal electric field, the equilibrium profile is maintained by a net EMF generated by tearing modes. These tearing modes are also expected to redistribute energy in the plasma through an outwardly-directed Poynting flux. In the experiments reported on here, insertable edge probes are used to measure the Poynting flux associated with coherent fluctuations in electric and magnetic fields. Our results indicate that this outward flux is a significant fraction of the total input power, peaking during discrete magnetic relaxation events (or sawtooth crashes). The flux is also observed to reach a maximum near the magnetic reversal surface, suggesting that electromagnetic energy is deposited there by nonlinear mode coupling. These results are consistent with estimates of global power balance from time-resolved experimental equilibrium reconstructions and are compared to expectations from fluid modeling of a resistive, incompressible plasma.