Laboratory Investigations of Magnetic Field Generation and Interactions Driven by High Power Lasers

Abstract

This thesis describes experimental and computational efforts to develop diagnostic tools, investigate magnetic field generation, and probe the dynamics of magnetic fields driven by high power lasers. Proton beams accelerated via the target normal sheath acceleration (TNSA) mechanism can be used to diagnose electric and magnetic fields present in high power laser-produced plasmas. In experiments using the OMEGA EP laser system, measurements were made of proton beams generated by up to 100 ps, kilojoule-class laser pulses having relativistic intensities. By systematically varying the laser pulse duration, degradation of the accelerated proton beam quality was measured as the pulse length increased. Two dimensional particle-in-cell (PIC) simulations and simple scaling arguments suggest that ion motion during the rise time of the longer pulses leads to extended preformed plasma expansion from the rear target surface and strong filamentary field structures which can deflect ions away from uniform trajectories, leading to large emittance growth. Optimal laser pulse conditions for proton radiography applications were identified. Proton radiography was used to explore both moderate and high intensity lasers interacting with foil targets. The strength, spatial profile, and dynamics of self-generated magnetic fields were measured as the target material was varied between plastic (CH), aluminum and copper. In the case of moderate intensity pulses, radiation-driven double ablation fronts in higher Z targets initiate multiple regions of Biermann battery magnetic field generation. Results were compared to extended magnetohydrodynamics (MHD) simulations with radiation transport which reproduced key aspects of the experiment. At high intensities, rapid expansion of the magnetic fields was observed, as well as enhanced filamentation in lower Z, insulator targets. 2D PIC simulations were used to probe the underlying physics of high intensity laser-driven magnetic field generation, including the impact of preformed plasma scale length. Finally, each of these elements were brought together to study a highly asymmetric laser-driven reconnection geometry established by focusing a high intensity pulse alongside a moderate intensity long pulse. After the long pulse plasma and associated magnetic field has evolved, the high intensity pulse arrives on target producing a relativistic, highly magnetized plasma. Proton radiography captures the dynamic interaction of the strong, impulsive magnetic field generated by the high intensity pulse and the relatively slowly evolving Biermann battery fields. Quantitative measurements of the magnetic field dynamics and 3D PIC simulation results show signatures of a magnetized interaction potentially indicative of shock formation and asymmetric magnetic reconnection.