Magnetic reconnection in the MST reversed field pinch.

摘要

Magnetic field line reconnection is a process whereby magnetic field lines which are otherwise topologically preserved by, and frozen into, a plasma can break and reconnect to form field lines with different topologies. It plays a significant role in a wide variety of plasmas, including stellar, space and laboratory plasmas. This dissertation focuses on the underlying dynamics of reconnection in one particular kind of laboratory plasma, the Reversed Held Pinch, where it takes the form of tearing mode activity. Specifically, it reports measurements of the spatial structure of tearing mode induced magnetic and parallel current density fluctuations in the edge of MST.; At least four significant results are obtained. First, we observe direct evidence of reconnection at the reversal surface of MST, in the form of a magnetic field fluctuation that causes reconnection there. Second, we observe the associated current sheet. Such current sheets are a characteristic feature of reconnection, but their thicknesses are sensitive to the specific physical effects that allow it to occur. We compare the thickness of the observed current sheet to expectations from models of reconnection that incorporate different physical effects, including, among others things, resistivity, electron inertia, electron pressure and ion inertia. Third, we obtain estimates of the radial current density due to streaming of charge carriers along magnetic field lines which have been perturbed by reconnection. We find that, in contradiction with the expectation for isolated tearing modes, it is non-vanishing, and large enough to imply the existence of some mechanism to maintain charge neutrality. Fourth, we measure the Lorentz force on the plasma associated with reconnection. We observe that, in agreement with expectation for interacting tearing modes, this force has a radial structure during sawtooth crashes that reduces the equilibrium toroidal velocity shear. We observe, also, that it is large enough to imply the existence of other forces on the plasma.