Shock Waves in Gas and Plasma in Strong Magnetic Fields
Shock waves in magnetized plasma are central to particle acceleration, high-energyradiation, and energy transport in astrophysical systems. However, direct measurements in the MG field regime relevant to white dwarf accretions, cataclysmic variables, and supernova remnants remain under tested. In this dissertation I develop an experimental platform combining a 50 TW ultrafast laser and a 1-MA pulsed-power generator to produce and diagnose laser-driven shocks propagating through gas and plasma in externally applied megagauss magnetic fields, a strong field regime not tested before. Experiments were performed at the Nevada Terawatt Facility(NTF). The platform uses the approximately 30 J, 0.35 ps-0.8 ns, 1056 nm Leopard laser focused onto 1-2.5 mm Cu rod loads positioned inside the Zebra pulsed-power vacuum chamber. A plasma piston produced by laser ablation drives a shock wave into an ambient hydrogen or nitrogen gas. Zebra drives 0.7-1.5 MA of current, generating azimuthal magnetic fields up to ≥ 100 T at millimeter distances from the rod surface. Magnetic fields were directly measured using two-color Faraday rotation with TGG crystal probes. Multi-frame optical diagnostics were developed and integrated into the Zebra chamber. This includes 266 nm interferometry, 532 nm shadowgraphy, schlieren imaging, 4-frame side-on imaging perpendicular to the rod, and vertical imaging parallel to the rod axis. These diagnostics enable time-resolved measurement of shock front position, curvature, density gradients, and velocity anisotropy, with temporal resolution on the order of several nanoseconds and μm spatial resolution. Detailed gas-jet density profiles between 1018 and 1019 cm-3 were calculated using Abel inversion algorithms. In shots without applied magnetic field, the shock expands quasi-spherically with measured front velocities in the range 500-1600 km/s. In the presence of MG magnetic fields, however, the dynamics change qualitatively. The shock front becomes anisotropic: radial expansion (perpendicular to B) is suppressed, while azimuthal expansion (parallel to B) remains significantly less affected. We additionally observe a high-speed azimuthal spread of plasma consistent with magnetic channeling of charged particles along field lines. In hydrogen, we directly measure an ionization wave ahead of the shock creating a cylindrical plasma channel simultaneously providing both gas and plasma shock medium. Simulations using 1-D Lagrangian HELIOS and 2-D HYDRA codes support the experimental interpretation and show that magnetic pressure B2/2μ0 modifies the effective shock jump conditions, when β ≤ 1. It's found with the tested experimental parameters the shock velcoity anisotropy scales with the magnetic field strength. The observed change to the shock symmetry can be explained with the inclusion of the magnetic pressure. This dissertation therefore presents laboratory measurements of laser-driven shocks in gas in MG magnetic fields, providing optical measurements of magnetic-field-modified shock geometry and velocity, and establishes a platform that can now access the field regime of intermediate polars. These results demonstrate that the transition from collisional to magnetically mediated shock structure can be studied in a controlled laboratory environment, and that laboratory plasma platforms can reach the magnetization needed to study astrophysical collisionless shock phenomena at previously unexplored field strengths.
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