Focusing and self-modified transport of high intensity proton beams

Summary form only given. Proton beams driven from short pulse lasers can deposit their energy in material with exceptionally high intensity and drive rapidly evolving plasma conditions and dynamic currents which lead to previously-unseen proton stopping and transport behavior. These effects, such as extended range and narrowing of the heated plasma volume, become more prominent as the beam energy and pulse length increase. To study this behavior we conducted experiments using kiloJoule short pulse lasers at the OMEGA EP facility. We used a 1250 J, 10 ps pulse to irradiate spherically curved diamond (C) targets with three target mounting configurations. The spectrum of protons from the target was measured from the source target as well as after passing through two transport layers and a Cu tracer layer. Cu K-shell x-ray fluorescence, caused by collisions of the beam protons and electrons with Cu atoms, was imaged with the Spherical Crystal Imager. The Cu K-α emission was focused and brighter by a factor of 8 with targets with conical structures compared to stand-alone hemispheres. The enhanced focusing and brightness is due to the sheath field generated inside the conical structure produced by fast electrons escaping the target. These focusing fields were detected using high spatial- and temporal-resolution proton radiographs. The beam density upon entry into the foils is estimated to be 10¹⁸ cm^-3 for the structured target. The proton spectra through the foils were quintessentially different for the structured (high intensity beam) vs. stalk-mounted (diverging beam) targets. We have developed the capability to model beam/solid interactions self-consistently by modification of the particle-in-cell code LSP to accurately predict collisional stopping of protons over a wide range of warm dense and hot dense states. The findings include rich new physical effects that become important in proton transport for beams corresponding to the experim- nt and hypothetical higher intensity beams.