Free electron laser probing of warm dense matter created by a laser-driven proton beam*

This covers our first experiment to study proton-heated Warm Dense Matter (WDM) with the Matter in Extreme Conditions (MEC) end station at the LCLS in SLAC National Accelerator Laboratory. The extremely high current-density proton beams that can be created with relativistically-intense lasers grant new capabilities in high energy-density science experiments by isochorically heating matter. Stopping powers for ions in WDM have large uncertainty, limiting our ability to predict the heating that could be achieved with above-mentioned proton beams in experiments and in fusion burn scenarios. To help benchmark our understanding of intense beam heating to the WDM regime, we carried out an experiment using the MEC’s 25 TW, 800 nm short pulse laser and the LCLS free electron laser (FEL). The short pulse irradiated a 5 μm thick Cu foil to create a proton beam that heated foil samples of 0.65 μm Al, 1.2 μm Al, or 4.0 μm polypropylene, which were then probed by the FEL operating at 7.5 keV. We present source and transmitted proton spectra as well as the first attempts to measure X-ray Thomson scattering from the heated volume to characterize its temperature and density.