Experimental Study of Magnetic Flux Transfer at the Hyperbolic Neutral Point

Abstract

A laboratory study is reported herein of a plasma flow process sometimes referred to as the severing and reconnecting of magnetic field lines at a hyperbolic or X-type neutral point. It is found that a hyperbolic pinch develops at the neutral point resulting in plasma compression. The Ohmic electric field of the pinch current enables field line reconnection to take place through resistive diffusion. The reconnection rate is substantially increased by a reduction in the size of the pinch region effected by flow adjustment through pairs of stationary slow mode shocks in a manner similar to that deduced and described by Petschek. Strong evidence is presented for this wave-dominated configuration. However, it is found that the pinch current becomes progressively concentrated at the neutral point and along the shock loci until eventually a critical current density is reached marked by a sudden and large anomalous increase in resistivity as predicted by Friedman and Hamberger. At this point the wave-assisted diffusion mode is terminated and there is observed: (1) a cutoff in pinch current, (2) a large increase in the electric field, (3) corresponding increase in reconnection rate, and (4) generation of a system of large amplitude fast-mode waves. The waves, bearing evidence of the onset of plasma turbulence, effect a rapid and gross redistribution of flux among the cells defined by the separatrix, and blow the slow mode shocks off downstream. The increased electric field raises the possibility of rapid magnetic energy dissipation through acceleration of particles to high energy.