Experimental Observation Of 3-D, Impulsive Reconnection Events In A Laboratory Plasma
Authors: S. Dorfman, H. Ji, M. Yamada, J. Yoo, E. Lawrence, C. Myers and T.D. Tharp
Abstract: Fast, impulsive reconnection is commonly observed in laboratory, space, and astrophysical plasmas. In this work,
impulsive, local 3-D reconnection is identified for the first time in a laboratory current sheet. The two-fluid, impulsive
reconnection events observed on the Magnetic Reconection Experiment (MRSX) [Yamada, et. al., Phys Plasmas 4, 1936] cannot be explained by 2-D models and are therefore fundamentally
three-dimensional. Several signatures of flux ropes are identified with these events; 3-D high current density regions
with O-point structure form during a slow buildup period that precedes a fast disruption of the reconnecting current layer.
The observed drop in the reconnection current and spike in the reconnection rate during the disruption are due to ejection
of these flux ropes from the layer. Underscoring the 3-D nature of the events, strong out-of-plane gradients
in both the density and reconnecting magnetic field are found to play a key role in this process. 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. An important discrepancy in the layer width and force balance between the collisionless regime
of MRX and Kinetic simulations is also revisited. The wider layers observed in MRX may 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 scale
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 the out-of-plane electron flow, ejecting flux ropes from the layer in a 3-D manner.
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Submitted to: Physics of Plasmas (November 2013)
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Download PPPL-5013 (pdf 1.97 MB 15 pp)
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