Experimental and numerical characterization of high-energy point-like x-ray sources for radiography applications at the LMJ-PETAL laser facility

We report on high-energy point-like x-ray backlighters generated with the short-pulse PETawatt Aquitaine Laser (PETAL) for the purpose of radiography experiments. 25-μm-diameter Ag wire backlighters were used to image 100-μm-thick Au slabs. Three backlighter configurations have been investigated in which the wire is either fixed on a substrate or left free-standing and irradiated either at maximum (5.6×1018 W/cm2) or reduced laser intensity (1.6×1018 W/cm2). The best spatial resolution (∼26 μm) and contrast (∼0.29) are obtained close to the radiography axis of the free-standing wire backlighter. A simulation tool chain, including a radiative hydrodynamics code, a particle-in-cell code, and a Monte Carlo code, has been developed to model the radiography experiment, from the interaction of the PETAL beam with the backlighter and the emission of x rays to the generation of the radiography image. The simulated x-ray spectrum includes emission from the Ag K-lines and a two-temperature bremsstrahlung background (∼0.1 and 3 MeV). In order for the simulations to reproduce the experimental data, we find that it is necessary to account for the laser-accelerated electrons (few hundred keV to few MeV) hitting the detector, which contribute as much as 75% of the measured signal for the high-intensity shot. These electrons are the main cause of degradation of the radiography contrast compared to predictions with x rays only.