High conversion efficiency and small spot size of Kα X-ray generated from nano-foam Cu targets irradiated by femtosecond laser pulses

Developing high-quality X-ray source to diagnose shock wave and changing the material and structure features of the target were presented to enhance its absorbability to ultra-intense laser energy. Experiments were carried out on the XingGuang-III Ti: sapphire laser facility (2.1– 6 J, 30 fs) at Laser Fusion Research Center, China Academy of Engineering Physics. The minimum intensity was 1.6 × 1018 W/cm2 on a nano-foam Cu target with the thickness of 100 µm, the porosity of 70% and the density ratio of 30% solids of Cu. The emission of Kα X-rays from the target was measured using a single-photon counting CCD device. This nano-foam target had generated a Kα peak photon rate of 2.9 × 108 photons sr−1 s−1 and the maximum conversion efficiency (CE) value was 0.0084%. The average CE of the nano-foam Cu was 1.8 times greater than that of foil Cu target. The minimum spot size of the X-ray source was measured to be about 40 µm at full width at half maximum, smaller than 47–86 µm of the foil Cu target using 0.1-mm thick knife-edge method. The nano-foam structure showed the potential of enhancing the CE of the femtosecond laser for X-ray conversion.