Magnetized, Laser-Driven, Plasma Experiments at Astrophysically Relevant Conditions and Proton Imaging of Magnetic Fields

Abstract

This thesis presents analysis relevant to two magnetized, astrophysically relevant experimental campaigns that were performed at the OMEGA laser facility. The magnetic fields of these systems were measured using proton imaging, in which high-energy protons interact with and are deflected by the electromagnetic fields, forming spatial variations in the proton fluence on the image plane. Proton images are determined by both the amplitude of the field and the orientation of the field relative to a proton's trajectory, which makes it difficult to generalize between different systems, although certain geometrical effects should always be considered.

The collisionless interaction of two counter-propagating, laser-irradiated plasmas results in the formation of small-scale magnetic filaments via the Weibel instability, a process which may explain the presence of magnetic fields throughout the intergalactic medium. Proton images of experiments studying this phenomenon display repeatable features corresponding to the filaments. Through analytical approximations and statistical analysis of synthetic proton images we determined that the observed images features fundamentally correspond to the transverse extent of the constituent filaments. How the image features related to the underlying field had not previously been understood.

The magnetic field produced by planets with active dynamos, like the Earth, can exert sufficient pressure to oppose inflowing, supersonic stellar wind plasmas. The effective obstacle to the flow in these systems is the pressure-balance surface between the stellar wind and the magnetic field, known as the magnetopause, and a standing bow shock forms at a standoff distance upstream from the magnetopause to redirect the flow. We performed scaled experiments to explore magnetized bow shocks, which consisted of a slow, low-density plasma flow impinging on the external azimuthal magnetic field around a current-carrying wire. We infer the presence of a shock at a significant standoff distance from the wire from the spatially resolved, optical, Thomson scattered spectra, and the inferred density jump suggests significant magnetization. We also observe the formation of a bow shock around the magnetized wire in proton images of the magnetic fields at 60, 70, and 80 ns after the initial laser drive for two different field amplitudes. Simulations of the experiment performed using the FLASH code supplement the data.