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/F 2 mixtures conventionally used in KrF amplifiers were recorded to find out the possibility for femtosecond pulse amplification at the broadband Kr 2 F (4 2 G ! 1,2 2 G) 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 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.Plasma channels produced by laser radiation in atmospheric air or some other gases are of great interest for many fundamental problems and technical applications. There are triggering and diverting of lightning [1,2], directing of microwave radiation to overcome its original divergence [3,4], laser-driven acceleration and guiding of electrons [5] among them.In contrast to early experiments with CO 2 laser pulses of microsecond length [6], where opacity of dense plasma produced via avalanche ionization limited the length and continuity of the channel, approaches based on the use of UV [1,2,7,8] or femtosecond [9,10] laser pulses can produce long-distance partially ionized tracks in air (or other gas) due to multiphoton ionization either with or without filamentation of radiation. Since primary photoelectrons are quickly recombined with positive ions and attached to the molecular oxygen (during the period of time ~10-50 ns), additional influence of longer laser pulse is anticipated to keep the electron density for much longer time [1]. There are several papers [11][12][13] in which combination of single fs and single ns pulses resulted in plasma revival and improved triggering and guiding electric discharges. However, they have dealt not with UV pulses but with near IR fs and visible or near IR-ns pulses. Application of a long train of ultrashort UV laser pulses or combination of such a train with a long UV pulse seems to be the most attractive for creation and further supporting of plasma channels [14]. In this paper we demonstrate that non-self-sustained electric discharge and electric breakdown are triggered and guided by a train of picosecond UV pulses overlapped with a long free-running

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 ( proportional, variantI(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- micros 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 technique using the broadband emission of a laser plume as probe radiation is applied to record UV-visible (190-510 nm) absorption spectra of Ne, Ar, and Kr, pure and in binary mixtures under moderate e-beam excitation up to 1 MW/cm(3). In all the rare gases and mixtures, the absorption spectra show continuum related to Rg(2) (+) homonuclear ions [peaking at λ∼285, 295, and 320 nm in Ne, Ar, and Kr(Ar/Kr), respectively] and a number of atomic lines related mainly to Rg(∗)(ms) levels, where m is the lowest principal quantum number of the valence electron. In argon, a continuum related to Ar(2) (∗) (λ∼325 nm) is also recorded. There are also trains of narrow bands corresponding to Rg(2) (∗)(npπ (3)Π(g))←Rg(2) (∗)(msσ (3)Σ(u) (+)) transitions. All the spectral features mentioned above were reported in literature but have never been observed simultaneously. Although charge transfer to a homonuclear ion of the heavier additive is commonly believed to dominate in binary rare-gas mixtures, it is found in this study that in Ne/Kr mixture, the charge is finally transferred from the buffer gas Ne(2) (+) ion not to Kr(2) (+) but to heteronuclear NeKr(+) ion.

Millimeter-long filaments and accompanying luminous plasma and defect channels created in fused silica by single, moderately focused femtosecond laser pulses with supercritical powers were probed in situ using optical imaging and contact ultrasonic techniques. Above the threshold pulse energy Eopt=5 μJ corresponding to a few megawatt power, the pulses collapse due to self-focusing and the nonlinear focus moves upstream with increasing pulse energy. Behind the focus, elongated, gradually narrowing awl-shaped channels of electron-hole plasma and luminescent defects are produced. In the channels, whose dimensions generally depend on the pulse energy, supercontinuum emission propagating downstream the channels occurs, although its observation requires elevated pulse energies above 25 μJ in order to compensate energy dissipation in the channels. Ultrasonic side-view imaging of the channels, conducted from a few millimeters distance, reveals predominantly compressive pressure transients. The compressive signals are observed above the same threshold pulse energy Eopt, and their amplitude increases linearly with the laser pulse energy, simultaneously exhibiting significant temporal broadening of the corresponding pulsewidths, reflecting square root dependence of the channel length and sublinear (∝E3/4) dependence of the source pressure on the pulse energy. Altogether, these optical and ultrasonic studies demonstrate filamentary pulse propagation with considerable dissipation (∼10 cm−1) in the awl-shaped subcritical plasma channels rich with generated point defects and optical damage sites.