UC San Diego

Intense proton beams are appealing research subjects in high energy density physics and fast ignition of inertial confinement fusion as they are advantageous to isochoric heating and local energy deposition deep in the medium. A leading method for generation of proton beams utilizes high-power lasers to accelerate protons to energies over MeV. For applications, generation of these intense beams must be controlled, and understanding of the physics of beam transport in a new intensity regime is required. This thesis contains experimental findings, as well as computational studies, on the generation of intense proton beams and their transport in solid density matter.

Experiments were carried out to compare proton beam focusing from different target geometries irradiated by an intense laser. Compared to a free standing target, enclosed targets show a narrower and brighter K-alpha radiation emission spot on a foil placed behind the target, indicating higher beam focusability. Numerical simulations have confirmed that the cause of the experimentally measured focusing effect is a field on the target enclosing structures. Furthermore, the long laser pulse duration (10 ps) was beneficial to keep providing radial electric fields for beam focusing.

This thesis presents a simulation of the transport and energy deposition for such an intense proton beam in solid-density matter, where both collective effects and the individual proton slowing-down are taken into account in a self-consistent, dynamically coupled manner. To achieve this, a new proton stopping power module covering warm dense matter states has been implemented in the hybrid PIC code LSP where the proton stopping power is updated with the varying local target thermodynamic state at each simulation grid and time step.

Detailed analyses were undertaken to comprehend the collective effects taking place in these system. As an example, a self-generated magnetic field can develop during beam transport at high current density. In the case of a narrow beam, it can be strong enough to pinch the beam, leading to the local target heating up to hundreds of eV.

Finally, simulations showing consistent results with experimental data demonstrate that varying stopping power in different materials during proton beam transport can significantly alter the target heating profiles.