ИСТИНА

Femtosecond laser plasma is a well-known effective source of high-energy particles and ultrashort hard x-ray radiation in the wide energy range. Such x-ray and gamma sources could have a large number of applications, for example in x-ray spectroscopy with high temporal resolution or some medical and biological research. Thus the increase of hard x-ray radiation yield from laser plasma is very interesting and important experimental problem. In our experiments we use liquid targets, namely the melted gallium targets. The impact of laser pulse with a short weak pre-pulse which is ahead of the main pulse for a few nanoseconds onto the surface of the melted gallium target could significantly increase the hard x-rays yield from laser plasma. This effect is associated with formation of dense micro-jets of matter above the target surface after the action of the pre-pulse. We studied micro-jets formation at various energies of the pre-pulse and various times after its action using optical pump-probe shadowgraphy. We were able to bind the micro-jets formation with features of the energy fluence in the focal spot of laser beam and make some assumptions about optimal parameters of the pre-pulse for the experiments on hard x-rays generation. Then we perform some experiments on x-ray spectroscopy of laser plasma source. Twofold increase of gamma radiation was observed when the energy of the pre-pulse (0.2 mJ) and delay time (11 ns) of the main pulse were close to their optimal values and intensity of the main pulse was about 2*10^18 W/cm^2. The maximal energy of gamma photons in this case increased from 200 keV without prepulse to 1125 keV with pre-pulse.