Magnetic Field Dynamics and Shock Physics at the High Intensity Frontier of Laser Plasma Interactions

Abstract

This thesis presents experimental, computational, and theoretical work studying shock formation and magnetic field generation. The work was performed primarily in the highly energetic relativistic regime of laser plasma physics and uses simulations to obtain insight into an even higher energy regime where quantum electrodynamic effects become important.

Two forms of collisionless shocks are studied: electrostatic shocks, and magnetized shocks. Electrostatic shocks are explored through a set of particle-in-cell simulations using a simplified two-dimensional slab model and simulations including a relativistic intensity laser pulse. These simulations were performed to understand the effects of multiple ion species on laser-driven shock formation and proton acceleration. It is found that with the inclusion of a second ion species multiple shocks can form and ions can experience more than one stage of acceleration. We attempted to modify standard shock theory, which treats ions as a fluid, to include a second ion species. However, it is shown that this fluid theory breaks down due to reflections which require a kinetic model. Laser driven simulations showed that the inclusion of a second ion species reduced proton acceleration. Double shocks are not always formed and require sufficient density steepening to form.

Magnetized shocks are studied through experiments and simulations. Using the OMEGA EP laser system, a plasma plume was generated on a thin plastic foil using a moderate intensity long pulse laser and strongly magnetized relativistic electrons accelerated from the focal spot of a relativistic short pulse laser were interacted with this plume. The interacting fields were measured using proton deflectometry. Forward scattering was used to model the observed features. The features could be formed due to a build up of short pulse generated fields, or annihilation of magnetic fields at the interface where the plasmas interact. A massive three-dimensional particle-in-cell simulation showed that a discontinuity was generated from which a shock is driven into the long pulse generated plasma plume. This discontinuity became unstable and two-dimensional simulations are used to understand the instability formation. Electron particle tracking shows trajectories consistent with shock drift acceleration. This is the first experimental evidence of semi-relativistic magnetized shock formation and may be the basis for many energetic experiments studying the microphysics of extreme astrophysical environments.

The final part of this thesis explores the generation of magnetic fields on the next generation laser facilities where expected intensities exceed 10^23 W/cm^2. At these intensities it has been shown that quantum electrodynamic effects appear, however it is unclear how these processes affect magnetic field generation in laser-solid interactions. By performing several two-dimensional particle-in-cell simulations at various laser intensities including QED effects, magnetic field generation was studied. It was found that the laser pulse channels and hole bores into the target, driving relativistic electrons along the surface of the target with an associated return current. The target electron density depletes due to this return current setting up fields that pull the radially expanding electrons towards the target, generating a thin layer of strong >0.1 MT magnetic fields. The scaling of these fields with intensity is found and is seen to be limited by radiation reaction as a significant fraction of energy is converted to photons. This field generation mechanism is modeled using a set of simple equations that allow for the estimation of maximum field strength using laser parameters.