MeV x-ray enhancement from Petawatt laser interaction with bare and bilayer 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 small source size (<100 µm), such energetic x-ray sources 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, ~4x10²¹ W/cm²) to study the production of secondary radiation from 1 mm thick Ta targets. Padding the target frontside with solid density plastic allows for greater target expansion due to the ~10⁹ intensity contrast ratio at 100 ps. This expansion enhances electron acceleration, providing an avenue to increasing the MeV x-ray dose by 75%. Measurements of electron spectra from micron-scale plastic and metal foils, coupled with MCNP transport calculations, support this finding.