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
$( < 100\ \mu\mathrm{m})$
, 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,
$\sim 10^{21}\ \mathrm{W}/\text{cm}^{2}$
) to study the production of secondary radiation from 1–5 mm thick Ta targets. The thickness scan, coupled with MCNP calculations, indicates that a few-mm target optimizes MeV x-ray dose while still preserving the small source size. In addition, favorable intensity scaling, along with padding the target frontside with critical density foam or metal nanostructures, provide avenues to increasing the x-ray dose by ~2-3x.