Time and spatially-resolved density measurement of Proton-heated Warm Dense Silica using Phase Contrast X-ray imaging

Understanding warm dense matter, a complex state of matter lying between condensed matter physics and plasma physics is highly relevant for a number of fields including modeling the evolution and structure of large planets and stellar surface transport properties.  The complicated interplay between physical processes in the WDM state make it particularly difficult to model theoretically. Furthermore, the extreme temperature-density space where WDM resides is challenging to create experimentally. Among all the methods, laser-produced proton heating has shown to be one of the best to generate uniform and solid-density warm sample. So far, the warm dense conditions achieved are typically inferred from changes in reflectivity or by changes in X-ray absorption (XANES), rather than measured. Here, we report experimental results obtained at the MEC end-station (SLAC), in which a 20 µm thick, 100 µm wide silicon target is heated by a broadband and energetic protons beam generated by a 15 TW short pulse laser. The density spatial and temporal evolution of the heated sample is measured through transverse phase contrast X-ray imaging using the 50 fs duration X-ray free electron laser operated at 11 keV. The ion beam spectrum is characterized using an absolutely calibrated Thomson parabola