The effect of laser pulse duration on proton radiography

Proton radiography is a valuable and popular diagnostic for measuring the quasi-static electromagnetic fields generated in high-energy-density (HED) experiments. The qualities of laser-driven, target-normal sheath acceleration (TNSA) proton beams have been previously demonstrated to have excellent laminarity and small virtual source size making them suitable for imaging. The broad-energy range of the TNSA protons allows time-of-flight chirping to observe the temporal evolution of fields on the $\sim 1$--$100$ ps timescales in a single shot and laser parameters influence the choice of optimal target thickness. Here, the OMEGA EP facility was used to experimentally consider the effect of the laser pulse duration (between 1 ps and 100 ps), intensity and target angle of incidence on the proton beam qualities of uniformity, image visibility, energy spectra and divergence. Two-dimensional particle-in-cell modeling examines the acceleration mechanisms to explain the degrading beam quality for longer pulse durations. These results will help guide future experimental and laser system designs.