Investigation of Boosted Proton Energies through Proton Radiography of Target Normal Sheath Acceleration Fields in the Multi-ps Regime

Multi-kilojoule, multi-picosecond short-pulse lasers, such as National Ignition Facility-Advanced Radiographic Capability (NIF-ARC) laser and the OMEGA-Extended Performance (EP) laser, which have been constructed over the last decade enable exciting opportunities to produce high-brightness, high-energy laser-driven particle sources for applications in high-energy-density (HED) science like proton fast ignition for inertial fusion energy. Recent results on these platforms have demonstrated enhanced accelerated proton energies and electron temperatures when compared to established scaling laws. Recent work has developed a new scaling for proton TNSA in the multi-ps regime. However, this new physics in the multi-ps regime motivates the need to understand the origin of this enhancement in proton energies. Towards this goal, this work presents the first measurements of the TNSA accelerating sheath field in the multi-ps regime for pulse durations from 1ps-30ps. This measurement was achieved by using a separate TNSA proton source to radiograph the spatio-temporal profile of the accelerating sheath that is responsible for proton acceleration. Use of stacked radiochromic film detectors allows for a discrete time profile of the radiographs thus enabling the measurement of a ``movie'' of accelerating sheath fields evolution. In performing this measurement, we extract quantities such as the sheath strength as a function of time and pulse duration. In addition, this work presents a novel preliminary methodology based on representation learning to integrate heterogeneous data, such as proton and electron spectra, to constrain parameters that are not directly measurable such as the spatio-temporal evolution of the accelerating sheath field.