The first stage of the petawatt excimer laser project started at the P.N. Lebedev Physical Institute, implements a development of multiterawatt hybrid GARPUN-MTW laser facility for generation of ultra-high intensity subpicosecond ultraviolet (UV) laser pulses. Under this project, a multi-stage e-beam-pumped 100-J, 100-ns GARPUN KrF laser was upgraded with a femtosecond Ti:Sapphire front-end, to produce combined subpicosecond/nanosecond laser pulses with variable time delay. Attractive possibility to amplify simultaneously short and long pulses in the same large-scale KrF amplifiers is analyzed with regard to the fast-ignition, inertial confinement fusion problem. Detailed description of hybrid laser system is presented with synchronized KrF and Ti:Sapphire master oscillators. Based on gain and absorption measurements at GARPUN amplifier and numerical simulations with a quasi-stationary code, we are predicting that 1.6 J can be obtained in a short pulse at hybrid GARPUN-MTW Ti:Sapphire/KrF laser facility, combined with several tens of joules in nanosecond pulse. Amplified spontaneous emission, which is responsible for the pre-pulse formation on a target, was also investigated: its acceptable level can be provided by properly choosing staged gain or loading the amplifiers by quasi-steady laser radiation. Fluorescence and transient absorption spectra of Ar/Kr/F₂ mixtures conventionally used in KrF amplifiers were recorded to find out the possibility for femtosecond pulse amplification at the broadband Kr₂F (4²Γ → 1,2 ²Γ) transition, which benefits in 100 times higher saturation energy density than for KrF (B → X) transition.

Non-self-sustained electric discharge and electric breakdown were triggered and guided by a train of picosecond ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire-KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only.

A new regime of the sliding-mode propagation of microwave radiation in plasma waveguides in atmospheric air is studied both experimentally and theoretically. The mechanisms of air photoionization and relaxation under propagation of 25-ns pulses of KrF laser are investigated. It is shown that a tubular plasma waveguide of large radius (much larger than wavelength of the microwave signal) can be produced in the photoionization of air molecules by 248-nm radiation of KrF-laser. We experimentally demonstrate the laser-enhanced transfer of 38-GHz microwave signal to a distance of at least 60 m. The mechanism of the transfer is determined by total internal reflection of the signal on the optically less dense wall of the plasma waveguide. Analytical and numerical simulations performed for various waveguide radii and microwave radiation wavelengths show that the propagation length increases with decrease in the wavelength reaching a few kilometers for submillimeter waves. Medium-size KrF laser facility with about 400-J energy in a train of picosecond pulses is suggested for the directed transfer of microwave radiation to 1-km distance.

Microscale optical breakdown induced in bulk pure water by high-power nanosecond KrF laser pulses was studied using optical transmission and contact broadband photoacoustic techniques. The breakdown has been identified as a sharp transmission drop coinciding with the appearance of unipolar compressive acoustic pulses, both indicating a thresholdlike rise of local intrinsic absorption in the micrometer-scale laser focal volume. The acoustic pulses, which are much broader than the exciting laser pulse and show a strongly reduced far-field diffraction effect, result from breakdown-induced millimeter-sized steam bubbles. The acoustic pulse amplitudes exhibit a sub-linear $(\propto I^{3∕4})$ pressure dependence on the laser intensity $I$ characteristic of subcritical electron-ion plasma and demonstrating the avalanche enhancement of two-photon ionization above the breakdown threshold until the appearance of the critical plasma. In the critical plasma regime, where the transmission and the acoustic signals slowly vary as a function of laser intensity, the main acoustic pulse is preceded by nanosecond and sub-$\mu \mathrm{s}$ prepulses, where the first one represents a GPa-level plasma-driven shock wave and the second one adjacent to the main pulse appears due to weak submillimeter-long heating of water surrounding the hot plasma by its bremsstrahlung radiation, indicating significant dissociation of water molecules in the plasma.

A new regime of the sliding propagation of microwave radiation in plasma waveguides in atmospheric air has been investigated experimentally and theoretically. A plasma waveguide whose radius is much...

Contact acoustic technique has been employed to perform spatially resolved in situ detection of microplasma formation and filamentation of tightly focused femtosecond laser pulses with supercritical pulse powers in bulk dielectrics, via corresponding acoustic emission. Investigation of acoustic generation mechanisms related to the plasma formation and filamentation effects reveals the critical character of the opaque microplasma and provides estimates of its gigapascal-level pressures and energy densities of a few $\mathrm{kJ}∕{\mathrm{cm}}^3$ . The acoustic measurement enables real-time in situ monitoring and revealing of basic mechanisms of ionization and filamentation in bulk dielectrics.