The input pulse of the laser PEARL with energy of 18 J and pulse duration of about 60 fs was compressed to 10 fs after passage through a 4-mm thick KDP crystal and reflection at two chirped mirrors with sum dispersion −200 fs 2 . The experiments were performed for the В-integral values from 5 to 19 without visible damages of the optical elements, which indicates that small-scale self-focusing is not a significant issue. It was shown that, by virtue of the low dispersion of the group velocity, the KDP crystal has some advantages over silica: a larger pulse compression coefficient, especially at a small value of the В-integral (B=5…9), lower absolute values of chirped mirror dispersion, and also a possibility to control the magnitude of nonlinearity and dispersion by changing crystal orientation.

A violation of the local relation between the visible continuum (VC) emissivity and the millimeter wave (MMW) intensity was observed in the experiments on imaging of MMW beams with subcentimeter width using the VC from a slab of the positive column (PC) of a medium-pressure cesium-xenon (Cs-Xe) dc discharge. The spatial distribution of the microwave-induced variation in the VC emissivity of the plasma slab was modeled. For this, perturbations of the electron temperature and density in a slab of the PC of a Cs-Xe discharge affected by an MMW beam were calculated. The relation between the spatial distributions of the VC brightness perturbation and the MMW intensity has been obtained. Good agreement between the results of the experiments and the modeling is demonstrated. The line spread function of a two-dimensional (2D) plasma sensor for MMWs is found, and its width is shown to be equal to 2 mm. We proved that the nonlocality of microwave-induced variations in the intensity of the VC from the PC, as well as the spatial resolution of the plasma technique of MMW imaging, are primarily determined by the influence of the electron heat conduction. Along with the electron heat conduction, the nonlocality of the electron temperature variation is caused by the influence of the hydrodynamic flux of electron enthalpy, diffusion and thermodiffusion fluxes of electrons, and also spatial inhomogeneity of the electron heating by a dc electric field, which results from a microwave-induced variation in plasma conductivity. These factors are responsible for the axial asymmetry of the images of the axisymmetric MMW beams.

It was shown experimentally that for a 65-fs 17-J pulse, the effect of filamentation instability also known as small-scale self-focusing is much weaker than predicted by stationary and nonstationary theoretical models for high B-integral value. Although this discrepancy has been left unexplained at the moment, in practice no signs of filamentation may allow the breakthrough in nonlinear pulse postcompression at high laser energy.

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A new front-end laser system with optical synchronization of chirped femtosecond and pump pulses for the petawatt laser complex PEtawatt pARametric Laser (PEARL) has been developed. The new front-end system provides a broader femtosecond pulse spectrum, temporal shaping of the pump pulse, and a significant increase in the stability of the parametric amplification stages of the PEARL.