Shock Structure and Radiative Cooling Effects on Reverse Shock Experiments at MAGPIE

Accretion shocks are ubiquitous phenomena in many astrophysical systems which can be significantly affected by radiative cooling effects [1-2], leading to instabilities and turbulence. In this study, we investigate the structure of reverse shocks resulting from the collision of supersonic, magnetized plasma flows generated by an inverse wire array interacting with a planar conducting obstacle.

Despite similar upstream flow velocities and mass densities, we observe striking variations in the reverse shock structure depending on the wire material used. Specifically, when aluminium wire arrays are employed, we observe a well-defined, sharp shock that aligns with magneto-hydrodynamic theory. However, in the case of tungsten wires, we do not observe a distinct stand-off shock; instead, we observe a broad region characterized by density fluctuations spanning a wide range of spatial scales.

These two contrasting interactions are diagnosed by using interferometry, Thomson scattering, shadowgraphy, and a newly developed imaging refractometer [3]. The refractometer enables to characterise the small-scale density perturbations by detecting subtle deflections of the probing laser. Our findings suggest that the differences in shock structure are most likely due to radiative cooling instabilities which give rise to density perturbations elongated along magnetic field lines. In aluminium plasma, these instabilities grow more slowly and are mitigated by thermal conduction.

[1] J. H. Hunter, Jr., “Generalized Thermal Stability and its Application to the Interstellar Gas”, ApJ, (1970).

[2] F. Suzuki-Vidal, et. al. “Bow Shock Fragmentation Driven by a Thermal Instability in Laboratory Astrophysics Experiments”, ApJ, (2015).

[3] Hare, J. D. et. al. “An Imaging Refractometer for Density Fluctuation Measurements in High Energy Density Plasmas”, RSI, (2020).

[4] S. Merlini, J. D. Hare, G. C. Burdiak, et. al. "Radiative Cooling Effects on Reverse Shocks Formed by Magnetised Supersonic Plasma Flows", AIP PoP, (2023) [arXiv preprint].