Plasmoid Formation and Strong Radiative Cooling in a Driven Magnetic Reconnection Experiment

We present the first experimental study of plasmoid formation in a magnetic reconnection layer undergoing rapid radiative cooling, a regime relevant to extreme astrophysical plasmas. Two exploding aluminum wire arrays, driven by the $Z$ machine, generate a reconnection layer ($S_L\approx 120$) in which the cooling rate far exceeds the hydrodynamic transit rate (${\tau }_{\text{hydro}}/{\tau }_{\mathrm{cool}}>100$). The reconnection layer generates a transient burst of $>1\mathrm{keV}$ x-ray emission, consistent with the formation and subsequent rapid cooling of the layer. Time-gated x-ray images show fast-moving (up to $50\mathrm{km}{\mathrm{s}}^{-1}$) hotspots in the layer, consistent with the presence of plasmoids in 3D resistive magnetohydrodynamic simulations. X-ray spectroscopy shows that these hotspots generate the majority of Al K-shell emission (around 1.6 keV) prior to the onset of cooling, and exhibit temperatures (170 eV) much greater than that of the plasma inflows and the rest of the reconnection layer, thus providing insight into the generation of high-energy radiation in radiatively cooled reconnection events.

Figure 1

(a) A three-dimensional model of the dual wire array load and diagnostic setup. (b) Current (purple), x-ray diode signal (green), and magnetic field at 5 and 10 mm from the wires (red). Inset: Simulated x-ray emission from the reconnection layer, filtered with $8\mathrm{\mu }\mathrm{m}$ Be, for non radiative (black) and radiatively cooled (orange) cases. X-ray power $P^{*}$ is normalized using the peak power in the radiatively cooled simulation.

Figure 2

(a)–(c) Time-gated x-ray images (10 ns exposure) of the reconnection layer, showing hot spots (green arrows) inside a bright elongated layer. (d) Synthetic x-ray image of the layer obtained by postprocessing a 3D simulation at 220 ns with x-ray emission and radiation transport modeling [36]. (e) Velocity of the hot spots inside the reconnection layer. The solid line is a linear fit to the hot spot velocity, and dashed orange line is the simulated outflow velocity in the reconnection layer, averaged between 200 and 250 ns.

Figure 3

(a) Time-integrated x-ray spectra from the reconnection layer showing Al $K$-shell emission. Red crosses show the position of the He-$\alpha$ intercombination line. (b) X-ray spectrum averaged over $z=(10\pm 1)\mathrm{mm}$ [white line in (a)] showing He-like and Li-like satellite transitions (black), and calculated spectrum from the SCRAM and radiation transport model for 170 eV, $1\times {10}^{18}{\mathrm{cm}}^{-3}$ hot spot density, and 0.5 mm hot spot size (gray dashed).