MeV x-rays from Petawatt laser interaction with flat, foam, and nanostructured targets

The interaction of an intense laser with overdense targets can readily generate MeV electrons, which in turn can be converted into MeV x-rays via the Bremsstrahlung process. With their ultrashort duration (~ps) and high resolution (<100 µm), such energetic x-rays have a wide variety of applications in both the dynamic and static radiography of dense materials. Here we report on results from the Texas Petawatt laser (120 J, 140 fs, ~5×10²⁰ W/cm²) to study the production of secondary radiation from thin (~µm) targets, which is subsequently injected into thick (~mm) targets. Multi-stage numerical modeling is required for good agreement, with particle-in-cell modeling predicting the electron acceleration, followed by Monte Carlo methods to simulate the conversion of MeV electrons into MeV x-rays. Experiment and simulations indicate that padding the mm-thick targets with critical density foam or nanostructures provides an avenue to brighter MeV x-ray beams, while showing that even high laser contrasts (>10⁹) must be accounted for.