Structure of accretion shocks and radiative cooling effects in high energy density plasma experiments
File(s)
Author(s)
Merlini, Stefano
Type
Thesis
Abstract
Accretion shocks are ubiquitous phenomena in many astrophysical systems which can be significantly affected by radiative cooling effects, leading to instability and turbulence. This thesis presents a study of accretion shock experiments at the MAGPIE pulsed power facility. Two methods are used to produce reverse shocks in the laboratory. The first investigates the structure of reverse shocks resulting from the collision of supersonic, magnetised plasma flows generated by an inverse wire array interacting with a planar conducting obstacle, whereas the second approach consists of the ablation by X-ray radiation from a Z-pinch wire array of solid targets. In the planar obstacle experiments, variations in the reverse shock structure are observed depending on the wire material used, despite similar upstream flow velocities and mass densities. Specifically, when aluminium wire arrays are employed, a well-defined, sharp shock is formed that aligns with magneto-hydrodynamic theory. However, in the case of tungsten wires, a distinct stand-off shock is not observed, instead, a broad region ahead of the obstacle is characterised by density fluctuations spanning a wide range of spatial scales. These two contrasty interactions are diagnosed with laser interferometry, Thomson scattering, shadowgraphy, and a newly developed imaging refractometer, enabling to characterise the small-scale density perturbations by detecting deflections of the probing laser. These measurements suggest that the differences in shock structure are most likely due to radiative cooling effects which give rise to density perturbations elongated along magnetic field lines. In the second experimental campaign, reverse shocks formed from the collision of counter-streaming supersonic plasma flows are investigated. The properties of the shocked layer were determined using various laser diagnostics and optical self-emission images, revealing overall consistency with a 1-D accretion shock model for a value of γ ≤ 1.2. In addition, the capability of the X-ray-driven platform to easily change the B-field orientation allowed us to explore the case for colliding flows in a transverse magnetic field, revealing promising results for investigating new astrophysical systems in the laboratory.
Version
Open Access
Date Issued
2023-10
Date Awarded
2024-02
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Lebedev, Sergey
Sponsor
Engineering and Physical Sciences Research Council
Department of Energy (U.S.)
Grant Number
EP/N013379/1
DE NA0003764
DE-SC00204
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)