Development of a compact laser-plasma hard x-ray source

Hard X-ray over 100 keV is known to be an important tool for nondestructive diagnosis of metal structures. These X-rays can be generated from radioisotopes such as ¹⁹²Ir or ⁶⁰Co, or from an X-ray tube. However, these isotopes require a severe control for a safely handling and the dimension of the X-ray tube is too large to measure in situ. The interaction of an ultraintense femtosecond laser pulse with matter acts as a source of hard X-rays produced by bremsstrahlung of relativistic electrons and characteristic line emission from the matter. The main drawback of this kind of X-ray source is the extremely high cost and the large scale of high-power femtosecond laser systems. In this study, we report on the development of a 104*82*75 mm³-sized X-ray source, which is driven by laser pulses of only 20 mJ energy and 40 fs duration. The laser beam with 12.7 mm diameter is delivered by small optics and is focused onto a tape target made of copper 5 mum thick. A measured electron temperature of energetic part was 340 keV, which agreed with that of two-dimensional collisional particle-in-cell simulations including plasma ionization. The irradiation size of the X-ray source at the laser focus was 16 mum measured by means of a knife-edge shadowgraphy. Using this compact X-ray source, a transmission image of a sample made of aluminum 10 mm thick was obtained with 100-shot laser accumulations. An estimated X-ray temperature of energetic part was 35 keV, which agreed with the electron temperature of low energy part.