Experimental Study of 3-D, Impulsive Reconnection Events in a Laboratory Plasma

Abstract

Fast, impulsive reconnection is commonly observed in laboratory, space and astrophysical plasmas. Many existing models of reconnection attempt to explain this behavior without including variation in the third direction. However, the impulsive reconnection events observed on the Magnetic Reconnection Experiment (MRX) which are described in this dissertation cannot be explained by 2-D models and are therefore fundamentally three-dimensional. These events include both a slow buildup phase and a fast current layer disruption phase. The buildup phase is characterized by a slow transition from collisional to collisionless reconnection and the formation of “flux rope” structures; these “flux ropes” are defined as 3-D high current density regions associated with an O point at the measurement location. In the disruption phase, the “flux ropes” are ejected from the reconnection layer as the total current drops and the reconnection rate spikes. Strong out-of-plane gradients in both the density and reconnecting magnetic field are another key feature of disruptive discharges; after finite upstream density is depleted by reconnection during the buildup phase, the out of plane magnetic field gradient flattens and this disruption spreads in the electron flow direction. Electromagnetic fluctuations in the lower hybrid frequency range are observed to peak at the disruption time; however, they are not the key physics responsible for the impulsive phenomena observed. Important features of the disruption dynamics cannot be explained by an anomalous resistivity model. Furthermore, an important discrepancy in the layer width and force balance between the collisionless regime of MRX and kinetic simulations persists when the fluctuations are small or absent, implying that they are not the cause of the wider electron layers observed in the experiment. These wider layers may instead be due to the formation of flux ropes with a wide range of sizes; consistent with this hypothesis, flux rope signatures are observed down to the smallest scales resolved by the diagnostics. Finally, a 3-D two-fluid model is proposed to explain how the observed out-of-plane variation may lead to a localized region of enhanced reconnection that spreads in the direction of electron flow.