Experimental results of radiative collapse in pulsed-power-driven magnetic reconnection*

We present results from the first experimental investigation of strongly-radiatively cooled magnetic reconnection, a regime relevant to extreme astrophysical objects, such as black hole coronae. In the experiment, two inverse wire arrays are driven by the Z machine (20 MA peak current, 300 ns rise time), generating oppositely-directed plasma flows with anti-parallel magnetic fields. Inside the reconnection layer which forms between the wire arrays, the radiative cooling time is much shorter than the hydrodynamic crossing time (τ_H/τ_cool ≈ 240), leading to a radiative collapse of the layer, which we observe using a suite of temporally and spatially resolved diagnostics.

 

We characterize the inflows to the reconnection layer using inductive probes and visible spectroscopy, and find that around the radiative collapse time, the flows advect B ≈ 3 T, with T_e ≈ 2-3 eV, and n_e ≈ 1×10¹⁸ cm^-3. We use a filtered X-ray diode to record >1 keV photons from the reconnection layer, and observe a narrow peak in X-ray flux (50 ns FWHM) before peak current, consistent with the formation and subsequent radiative collapse of the reconnection layer. Time-integrated, spatially resolved X-ray spectroscopy of the layer shows clear He-like and Li-like K-shell emission lines, consistent with temperatures of about 175 eV. Through radiation transport calculations, we find that the X-ray spectrum is best described by localized dense hotspots embedded within a colder, less dense layer. This is consistent with results from two gated X-ray cameras, which observe an elongated interaction region with brightly-emitting hotspots traveling at up to 60% of the simulated magnetosonic velocity. Resistive MHD simulations indicate that hotspots are consistent with plasmoids, which are sites of enhanced emission.