Study of Proton Heating in Proximally Structured Cu Foam Targets

Fast Ignition offers the potential for higher gains by driving the compression and ignition phases separately to reduce laser driver energy and symmetry requirements. Proton beams are better suited than electron beams to heating the pre-compressed fuel because they can be tightly focused, rapidly depositing energy locally with minimal transport instabilities. A proton-driven Fast Ignition experiment was conducted to study proton heating in proximally structured Cu foam targets. Protons were accelerated from a curved foil using the LFEX laser and after transporting through a gold cone, the focused proton beam was delivered to a Cu foam cylinder. Free-standing foil and foil-in-cone targets were also tested without the cylinder. One x-ray spectrometer, two x-ray imagers, and three proton spectrometers were fielded for this experiment. An order of magnitude fewer protons with ~10 MeV lower cutoff energies were detected using the foil-in-cone-target relative to the free-standing foil. A comparison of the foil-in-cone proton spectra with and without the cylinder shows that the cylinder absorbed a majority of on-axis proton energy, and comparing the experimental x-ray spectrum to simulation suggests a cylinder temperature exceeding 200 eV. Simulated Kα emission from the LSP code agrees with experiment and explains that observed x-ray emission features are due to differences in proton and electron stopping powers.