Local measurements of tearing mode flows and the MHD dynamo in the MST reversed-field pinch

We investigate the spatial structure and consequences of tearing mode flows in the core of a Reversed-Field Pinch RFP by nonperturbing localized measurements of the impurity ion flow. The flows resulting from tearing modes play a fundamental role in the RFP including the generation of the magnetohydrodynamic (MHD) dynamo ⟨v˜ x b˜⟩ through correlations with magnetic field fluctuations. Carbon emission from neutral beam induced charge exchange recombination is collected by a novel high optical throughput duo spectrometer providing localized (+/- 1 cm) measurements of C+6 impurity ion velocities with high bandwidth (100 kHz). Correlations with dominant magnetic modes yield the first localized measurements of tearing mode flows in the core of a hot plasma. Poloidal tearing mode flow profiles resolved to better than 500 m/s in the Madison Symmetric Torus reveal strong correlations for a range of m = 1 magnetic modes. Tearing mode flows are observed to be localized to the resonant surface with flow layers much broader than predicted by linear MHD theory, and comparable to other characteristic scale lengths, such as the magnetic island width and ion skin depth. The relative poloidal flow amplitudes among the dominant core modes do not reflect the proportions of the magnetic amplitudes. During low tearing activity, the largest correlated flows are measured in association with the smaller amplitude magnetic modes resonant in the mid-radius. The measurements suggest that parallel tearing mode flows may be comparable to the perpendicular component. During periods of intense reconnection activity correlated flow amplitudes increase substantially and broaden in radius. Calculation of the MHD dynamo from poloidal tearing mode flow profiles yields the complete MHD dynamo on the magnetic axis. The MHD dynamo on the magnetic axis is adequate to balance the mean parallel Ohm's law during low tearing activity and is significant but does not exclude other dynamo mechanisms from contributing during a surge in reconnection activity. Contributions to the MHD dynamo from poloidal flows off axis are large when compared to the total dynamo action on axis.