Препринты / Preprints

The first stage of Petawatt Excimer Laser Project started at P.N. Lebedev Physical Institute implements a development of multiterawatt hybrid GARPUN-MTW laser facility for generation of ultra-high intensity subpicosecond UV laser pulses. Under this Project, a multi-stage e-beam-pumped100-J, 100-ns GARPUN KrF laser was up-graded with a femtosecondTi:Sapphire front-endto produce combined subpicosecond/nanosecondlaser pulses with variable time delay. Attractive possibility to amplify simultaneously short & 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. Gain and absorption measurements at GARPUN amplifier, being compared with numerical simulations based on a quasi-stationary code, precede amplification of short pulses in large-aperture e-beam-pumpedmodules. 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 stage 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 (42№ ??>1,2 2№ ) transition. Numerical simulations predictthat 1.6 J can be obtained in a short pulse at hybridGARPUN-MTWTi:Sapphire / KrF laser facility combined with several tens of joules in nanosecond pulse.

Keywords:

e-beam-pumped high-power KrF laser system with short-pulseTi:Sapphire front-end,generation of ultra-high intensity laser pulses.

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In this work we propose two neutral pion photoproduction experiments near reaction threshold. Using lH2 and lD2targets we can obtain the best world data

for TT0-photoproduction. From these data we can extract the threshold ?o+ (TT0n) amplitude for neutral pion photoproduction with high precision to test the self-consistency of Low Energy theorems (LET) for different channels of TT0-photoproduction and to testOCD-based theoretical predictions and, in particular, chiral perturbation theory (ChPT).

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A direct relativistic electron acceleration simulation model with a given electromagnetic field, which is determined by the wave packet parameters, is considered. The multimode time-spatial structure of a focused Nd-laser beam with stochastic phase disturbances of each of the spectral components is taken into account as a source of random forces. The electron energies of more than 10 MeV are derived even at moderate flux densities of 1016 W/cm2. The developed numerical code makes it possible to obtain a quantitative energy distribution function in relation to both the field intensities and temporal U-shape of the laser pulse. An efficient heating of electrons can be triggered in the presence of counter propagating wave being reflected from the critical plasma area with varied reflection coefficient. The heating mechanism occurs with a delay relative to the beginning of a pulse when the laser fields exceed certain threshold amplitudes. A qualitative comparison of simulation results with the experimental data is given as the evidence that this mechanism is not unreasonable.

Text of the report presented to 3rd International Conference on the Frontiers of Plasma Physics and Technology (FPPT-3), March 5-9, 2007, Bangkok, Thailand.

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