Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment

We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. Oppositely directed magnetic fields ($B=3\mathrm{T}$), advected by supersonic, sub-Alfvénic carbon plasma flows ($V_{\mathrm{in}}=50\mathrm{km}/\mathrm{s}$), are brought together and mutually annihilate inside a thin current layer ($\delta =0.6\mathrm{mm}$). Temporally and spatially resolved optical diagnostics, including interferometry, Faraday rotation imaging, and Thomson scattering, allow us to determine the structure and dynamics of this layer, the nature of the inflows and outflows, and the detailed energy partition during the reconnection process. We measure high electron and ion temperatures ($T_e=100\mathrm{eV}$, $T_i=600\mathrm{eV}$), far in excess of what can be attributed to classical (Spitzer) resistive and viscous dissipation. We observe the repeated formation and ejection of plasmoids, consistent with the predictions from semicollisional plasmoid theory.

Figure 1

(a) Experimental setup with the geometry of the reconnection layer. The cutaway on the right array shows the current path. (b) Top view with density map (taken at $t=272\mathrm{ns}$ after current start) and Thomson scattering vectors. (c) Side view interferogram.

Figure 2

Electron density maps from laser interferometry, both from the same shot. (a) At 223 ns after current start. (b) At 243 ns after current start. In both (a) and (b) there is an obvious region of enhanced density (a “plasmoid”) inside the reconnection layer. (c) Lineouts of electron density; $x$ positions shown in (a). (d) Lineouts of electron density across the reconnection layer; $y$ positions shown in (b).

Figure 3

Data from Faraday effect polarimetry, taken at $t=251\mathrm{ns}$ after current start. (a) Rotation angle of the linear polarization of the laser beam passing in the $y$ direction. (b) Measured magnetic field profile (blue) and Harris sheet fit (red). (c) Electric current density calculated from the Harris sheet fit.

Figure 4

Thomson scattering measurements taken at $t=232\mathrm{ns}$ after current start. (a) Inflow velocity. (b) Electron and ion temperatures. Spatial error bar shown for the first data point. (c) Calculated power flow into and out of the reconnection layer.

Figure 5

Plasmoid formation and dynamics in three optical self-emission images from the same experiment: 5 ns exposure, 20 ns between frames. The location of one plasmoid in each frame is indicated with a white arrow.