First Shadowgraphy Measurements of a Radiatively-Cooled Magnetic Reconnection Layer on the Z machine
Lansing Horan IV, Massachusetts Institute of Technology
Shadowgraphy is the technique of shining a laser beam through a plasma to infer electron density information. Gradients in the plasma's refractive index (which depends on the electron density) perturb the beam photons as they traverse the medium. The shadowgraphic image is a spatial intensity mapping of the exiting beam profile as resulting from photon deflections. This diagnostic was recently fielded on the fourth shot on MARZ (Magnetic Reconnection on Z), a high-energy-density experimental campaign on the Z Machine at Sandia National Laboratories. MARZ is a pulsed-power platform for studying magnetic reconnection in the presence of strong radiative emission, as relevant to extreme relativistic astrophysical environments. MARZ4 marks the first time that shadowgraphy has successfully imaged a radiatively-cooled reconnection layer. Four images were obtained, spaced 12 ns apart with 3 ns exposure; these images show evolution in time of the layer at snapshots before, during, and after peak x-ray emission. The experimental images contrast with synthetically-produced shadowgraphs. The dual exploding aluminum wire array load hardware for MARZ is simulated in the resistive magnetohydrodynamic (MHD) code, GORGON. Monte Carlo ray-tracing calculates the paths of millions of photons (rays) and their continual deflections as they transport through 3D electron density meshes; then, ray-transfer matrix analysis is performed to propagate these rays through a virtual optic and onto an imaging plane. Both the experimental and synthetic images present the reconnection layer as a dark central void; however, intensity pile-up (due to MHD shocks) and fine-scale structures differ in appearance, perhaps due to: differences in spatial resolution, the inherently random patterns of instabilities, simulations missing some underlying physics, or other effects. Analysis efforts are currently underway to extract quantitative information from the shadowgraphs.
Authors: Lansing S. Horan IV, Katherine Chandler, Rishabh Datta, David Yager-Elorriaga, Jack D. Hare & the MARZ Collaboration