Tunable MeV x-rays from Petawatt laser-solid interaction*

It is well established that high irradiance lasers (>10¹⁸ W/cm² μm²) can drive MeV x-rays, essential tools for nuclear physics, nuclear medicine, and high-energy radiography. However, for  tunable source generation, open physics questions remain, such as the role of focal geometry and relativistically induced transparency (RIT). A focal geometry study (f/1.5, 10²¹ W/cm² and f/3, 5×10²⁰ W/cm²) at Texas Petawatt (120 J, 140 fs) was carried out, with the laser incident on 1 mm Ta target to produce bremsstrahlung x-rays. The x-rays were characterized with an image plate stack known as a Filter Stack Spectrometer (FSS). Despite the lower intensity, the f/3 produces 3× greater MeV x-ray dose than the f/1.5. Particle-in-cell simulations indicate that the larger interaction volume allows for 3× greater coupling efficiency to MeV electrons. The simulations also convey that the f/1.5, due to its higher intensity, undergoes RIT in a target expanded by preplasma. For this system, coating the 1 mm Ta with microns of plastic increased the experimental MeV x-ray yield by 60%.