The paper presents the results of theoretical and experimental studies of the second harmonic generation process in the Ti:sapphire femtosecond complex, which includes a generator of the femtosecond pulse, stretcher, regenerative amplifier, two multi-pass amplifiers, compressor and second-harmonic generator. This complex provides the 50-fs pulses with energy of 20 mJ and it is used as a master oscillator in THL-100 hybrid laser system, which operates in the visible region at a wavelength of 475 nm. Experiments and calculations for various beam parameters of the fundamental harmonic, such as radiation intensity, spatial profile of the beam and the level of the noise component were performed. It is theoretically shown that in the absence of the noise component in the beam of the fundamental wave a good uniformity of the second harmonic should be observed. When making the amplitude heterogeneities in the first harmonic even greater heterogeneities in the second harmonic are appeared. It is experimentally shown that with increasing of energy beam the inhomogeneity of the second harmonic beam increases.
In this paper we discuss laser wake field acceleration experiments made at the PEtawatt pARametric Laser (PEARL).
Using free 2mm and 5 mm gas jet without preplasma we generated electron beam with energy up to 260 MeV. The
charge of quasimonoenergetic beams achieved 300 pC, angular divergence 0.2 degree. The special attention is paid for
diagnostics which is adapted for low repetition rate systems with low output parameters stability.
Theoretical investigation of second harmonic generation (SHG) of super intense femtosecond radiation demonstrated
that temporal Intensity Contrast Ratio (ICR) of output radiation of petawatt level laser complexes can be significantly
increased. The cubic polarization effects in the process give possibility of additional pulse compression. We present
experimental results of SHG of radiation with average intensity 2TW/cm<sup>2</sup> in 0.6mm KDP crystal. Theoretical model of
linear regime of plane wave instability in mediums with quadratic and cubic nonlinearity is developed and thoroughly
discussed. Analysis of small-scale self-focusing suppression methodic is presented. The influence of surface dust to
spatial noises generation in SHG process is pointed out.