Proton radiography of indirect-driven implosion by high-intensity laser generated protons on SGⅡ-U laser facility

A preliminary experimental study of the indirect-driven implosion by means of proton radiography has been carried out on the SGⅡ-U laser facility. In the experiment, the probing protons were generated by a picosecond laser irradiating on a thin Au target via the target normal sheath acceleration scheme. Maximum proton energy up to 18 MeV was achieved as a result of optimizing the laser and target parameters. In the static imaging of objects, the spatial resolution of proton radiography was demonstrated to be better than 20 m. In the time-resolved imaging of the implosion, the proton radiographs of the capsule at various stages of compression were obtained. It is found that the probing protons were expelled from the center region, which, covered an area much larger than the size of the compressed capsule. The detailed analysis reveals that the protons are expelled by the surrounding electric-magnetic fields as well as by the compressed capsule itself. It is also found that the energy of probing protons decreased when the nanosecond laser was present in the experiment. This is because the X-ray, generated from the nanosecond laser irradiation on targets, would partially destroy the rear side of proton target, thus leading to the less efficient proton acceleration. To improve the proton energy, the shielding design of proton target should be further considered in the next experiments.