Bow shock interaction experiments in a magnetised collisional plasma

Abstract

This thesis reports on shock interaction experiments at the MAGPIE pulsed power facility. Bow shocks are produced by placing cylindrical obstacles into the supersonic, super- Alfvénic and super-fast-magnetosonic plasma outflow from an inverse wire array z-pinch. The wire array provides a continuous source of plasma for ~ 500 ns with a velocity of ~ 50 km/s which advects a dynamically significant magnetic field of ~ 1.5 - 2.5 T. The geometry of the experiment allows good diagnostic access to the bow shocks and obstacles are fielded in pairs so that neighbouring bow shocks interact. The complex shock structure which emerges is a result of the resistive diffusion length being much larger than the collisional mean free path. Two types of shock are seen. One is characterised by a large shock transition length and very little heating and is a sub-critical resistive dissipative shock. This is observed to form as a single shock across both obstacles. The second are two shocks which form close to the obstacles and are downstream of the resistive dissipative shock. These exhibit substantial heating and have a shorter transition length. We interpret these to be hydrodynamic-like. The experiments are diagnosed with laser interferometry, Faraday rotation imaging, optical Thomson scattering and optical self-emission imaging which provide detailed time and space resolved measurements of the plasma parameters, including inside the resistive dissipative shock transition. An experimental validation of the Faraday rotation imaging diagnostic is presented to evaluate analysis techniques. Experiments with different plasma materials are compared, which demonstrates the dependence of the resistive dissipative shock transition length on the resistive diffusion length. The experimental results are compared with AstroBEAR simulations including new simulation results which include a magnetic field in such systems for the first time. Preliminary results show good agreement with our interpretation of the experiments.