ИСТИНА

Laser plasma produced by a high-power femtosecond laser pulse onto the surface of metal targets is known as an effective source of intense x-ray radiation in a wide energy range. The uniqueness of such a source is in simultaneous combination of short x-ray pulse duration (of about several picoseconds), capability of spectrum control, small x-ray source size. These aspects allow to use such x-ray source in x-ray spectroscopy with high temporal resolution, EXAFS spectroscopy, diffraction analysis, nuclear spectroscopy, medical and biological research, etc. In this paper we present the results of experimental investigation of spectral range and spatial size of the x-ray radiation source created by sub-relativistic femtosecond laser pulse onto the surface of the melted gallium target. Plasma was created by laser pulse delivered by Ti:Sa laser system (wavelength – 805 nm, pulse duration – 55 fs, pulse energy – 8 mJ, repetition rate – 10 Hz). Radiation was focused onto the surface of the melted gallium target under the angle of 60 degrees to the normal to the target surface by the Al off-axis parabolic mirror with a focal length of 5.1 cm. It provided intensity on the target surface up to 7*10^19 W/cm^2. The temperature of melted gallium was 152 degrees Celsius and continuously controlled by a thermocouple. The x-ray CCD Princeton Instruments PI-MTE was used to measure the x-ray emission spectrum. This tool allows to measure the x-ray radiation in the spectral range from 30 eV to 10 keV. The pinhole was used to estimate the spatial size and extent of the spatial coherence of the source and also to obtain the images of the source micro structure. To create the pinhole the 10 um and 20 um diaphragms were used. In addition, during the experiment the yield of x-ray radiation was continuously measured by two photomultiplier tubes FEU-119 with NaI(Tl) scintillator. These measurements allow to independently identify the temperature of the hot electron component in each laser shot. The temperature of hot electrons was about 40 keV. This suggests the significant influence of the source micro structure on the mechanisms of electron acceleration and yield of x-ray radiation.