High conversion efficiency and small spot size of Kα X-ray generated from nano-foam Cu targets irradiated by femtosecond laser pulses
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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.
We wish to thank all crew of the laser operation and the experiment groups on XG-III at CAEP. This work was supported by the National High Technology Research and Development Program of China, State Key Laboratory of High Field Laser Physics (Shanghai Institute of Optics and Fine Mechanics, CAS), Scientific & Technological Research Program of Chongqing Municipal Education Commission (KJ1600633), the National Natural Science Foundation of China (Grant No. 11504421) and Chongqing Key Laboratory of Manufacturing Equipment Mechanism Design and Control (KFJJ2016031, KFJJ2017052 and KFJJ2017053).
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