A new scheme providing self-compression of femtosecond laser radiation upon filamentation of a collimated beam in argon at a pressure of ∼0.8 atm is experimentally realised. Pulses with the initial duration of 55 fs were compressed down to 8 fs, the peak power of the compressed pulse exceeding 20 GW.

The dependence of the parameters of femtosecond laser radiation (55 fs, 3.7 mJ, 800 nm) in a filament on the distance along the filament propagation axis, the diaphragm diameter, and the type and pressure of gas used for filamentation has been studied experimentally.

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance.

The peculiar properties of plasma formation on the surface of different liquid metals by femtosecond laser radiation have been studied. It is shown that plasma formation and generation of hard X-radiation on the surface of molten metal depends substantially on the contrast and weakly on the polarization of laser radiation, which clearly distinguishes the plasma produced in our experiments from the plasma generated on the surface of a solid target.

It was shown that metals such as gallium, indium and bismuth heated above melting temperature can be used as a target for 10 Hz femtosecond laser plasma highly stable hard x-ray source.

Experimental results are presented which demonstrate that a plasma produced on the melted-gallium surface by a femtosecond laser pulse of intensity above 1016 W cm-2 is an efficient and stable source of incoherent hard X-rays with a pulse repetition rate of 10 Hz. For gallium heated up to 270°C, the X-ray yield decreased by ∼25% [from (2.2 ± 0.4) × 10-4 % to (1.7 ± 0.4) × 10-4 %] after 50000 laser shots, while the average energy of hot electrons decreased from 9.3 ± 0.9 to 9.0 ± 1.1 keV.

This study is devoted to investigation of femtosecond laser radiation parameters (intensity, contrast, polarization) influence on properties of hot electron population of dense plasma created at the surface of melted metal target.

It was shown that liquid gallium heated up to 270 degrees C can be used as a target for 10 Hz femtosecond laser plasma highly stable x-ray source. The decreasing of hard x-ray yield during 50000 laser shots is less then 25% and can be easy compensated by additional focusing of objective or temperature tuning.

Experimentally it was shown that liquid gallium can be used as a target for 10 Hz femtosecond laser plasma x-ray source. X-ray measurements demonstrated that temperature of hot electrons in gallium plasma does not differ from plasma of solid target and equal to 6.2 +/- 1.0keV.