Radiatively Cooled Magnetic Reconnection Experiments Driven by Pulsed Power

Abstract

Magnetic reconnection is a ubiquitous process in astrophysical plasmas, responsible for the explosive conversion of magnetic energy into thermal and kinetic energy. In extreme astrophysical systems, such as black hole coronae and neutron star magnetospheres, radiative cooling modifies the energy partition by rapidly removing internal energy. In this thesis, we perform experimental and computational studies of magnetic reconnection in a radiatively cooled regime, previously unexplored in reconnection studies. The Magnetic Reconnection on Z (MARZ) experiments consist of a dual exploding wire array, driven by a 20 MA peak, 300ns rise time current generated by the Z pulsed-power machine (Sandia National Labs). The load generates oppositely-directed supersonic, super-Alfvénic, collisional plasma flows with anti-parallel magnetic fields, that generate a reconnection layer (Lundquist number SL ∼ 100), in which the total cooling rate far exceeds the Alfvénic transit rate [mathematical notation].

Two- and three-dimensional simulations of the MARZ experiments are performed in GORGON, an Eulerian resistive magnetohydrodynamic code. The simulations demonstrate the generation of a reconnection layer, which radiatively collapses, exhibiting a rapid fall in temperature, strong compression, and an increased reconnection rate consistent with theoretical predictions. The reconnection layer is unstable to the plasmoid instability, generating secondary current sheets separated by magnetic islands. High energy X-ray emission is generated predominantly by the plasmoids. The plasmoids also collapse radiatively, and the reconnection layer recovers a laminar large aspect ratio structure, which does not exhibit further plasmoid generation, indicating stabilization of the original plasmoid instability of the current sheet.

The experiments confirm numerical predictions by providing evidence of plasmoid formation and strong radiative cooling. Experimental diagnostics directly measure the spatial, temporal, and spectral properties of radiative emission from the reconnecting system. The reconnection layer generates a transient burst of >1 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 km s−1) hotspots in the layer, consistent with the presence of plasmoids in 3-D resistive magnetohydrodynamic simulations. X-ray spectroscopy shows that these hotspots generate the majority of Al K-shell emission (around 1.6 keV), and exhibit temperatures (170 eV) much greater than that of the plasma inflows and the rest of the reconnection layer.

The findings in this thesis are of particular relevance to the generation of radiative emission from reconnection-driven astrophysical events, and to the global dynamics of reconnection in strongly cooled systems. The MARZ experiments also provide a novel platform for investigating radiative effects in high-energy-density and laboratory astrophysics experiments, and for validation of radiation magnetohydrodynamic and atomic spectroscopy codes.