Review of controlled laboratory experiments on physics of magnetic reconnection

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Abstract
We review results from the most recent experiments in the past 2 decades in which magnetic reconnection has been generated and studied in controlled laboratory settings. As a whole, research on the fundamental physics of the reconnection process and its hydromagnetic consequences has been largely theoretical. Laboratory experiments are crucial for understanding the fundamental physics of magnetic reconnection since they can provide well‐correlated plasma parameters at multiple plasma locations simultaneously, while satellites can only provide information from a single location at a given time in a space plasma. Thanks to the significant progress of data acquisition technology, the detailed magnetic field structures of the reconnection regions were measured and plasma acceleration and strong ion heating were identified. Extensive data have been accumulated in the electron MHD plasma regimes [in which electrons are magnetized] with relatively low Lundquist number of S = 1 – 10 as well as in MHD plasmas with S = 100 – 1000. This article puts a special focus on the most recent plasma merging experiments in the MHD regime since they are new and have been published in the past several years. Two distinctly different shapes of diffusion regions were identified both in the electron MHD plasma regime [Stenzel and Gekelman, 1983] and in the MHD plasmas in MRX [Yamada et al., 1997b]. The familiar two‐dimensional (2‐D) feature, a double‐Y‐shaped diffusion region, was identified when there is no or very little axial magnetic field (third vector component). In cohelicity merging with a sizable third field component, an O‐shaped diffusion region appears and the reconnection rate decreases substantially; this is attributed to the axial field pressure and the incompressibility of the plasma. Another important achievement is the experimental verification of a generalized Sweet‐Parker model [Ji et al., 1998]. In this recent work it is found that the observed reconnection rate can be explained by a generalized Sweet‐Parker model, which incorporates compressibility, downstream pressure and the effective resistivity. The latter is often enhanced over its classical values in the collisionless limit. A significant implication of this result is that the generalized Sweet‐Parker model is valid in certain 2‐D reconnection cases with axisymmetric geometry. In MHD plasmas, the thickness of this thin current layer is found to be of the order of the ion gyroradius and the ion skin depth as well.