experimental and numerical investigations of laser annealing of thin ferroelectric heterostructures have been presented in this paper. Laser annealing of thin films has been made in KrF laser facilities (Lebedev Phys. Institute, Moscow). 2D numerical code "TEPOL" have been developed for modeling of physical processes in multilayer samples. X-ray structural analysis and e-beam microscopy have been used to research the samples. The laser heating leads to partial crystallization of film and considerable change metal system in compare with traditional heat treating. A new approach to the study of samples with help of photo-acoustic method has been discussed in the paper.
The physical-mathematical model and 2D code "TEPOL" are developed to simulate the laser annealing of thin electroceramic samples. The results of test problem calculations have been presented in this paper. We have discussed the opportunity to improve the homogeneously of sample heating by using the transparent cover for ferroelectric
heterostructures.
Hydrodynamic regimes of KrF laser interaction with solid and thin-film targets in atmospheric and reduced pressure air were investigated at high-energy GARPUN installation. These experiments were performed with 100-J, 100-ns laser pulses in planar focusing geometry and compared with numerical simulations with ATLANT code to verify the concept of laser-driven shock tube (LST), which could accelerate a gas to hypersonic velocity and produce strong shock waves (SW). Laser beam was focused by a prism raster optical system that provided very uniform intensity distribution at moderate laser intensities q ≤ 1 GW/cm2 over a square spot of ~ 1-cm size. Dynamics of laser-produced plasma and SW in a surrounding gas were investigated by means of high-speed photo-chronograph and streak camera in combination with shadow or schlieren techniques, time and space resolved spectroscopy in a visible spectral range. Both experiments and simulations confirmed that target evaporation and blow-up of expanding plasma are the main mechanisms of UV laser-target interaction in a surrounding gas. Planar shock waves with velocities up to 7 km/s towards the laser beam were observed in a normal density air and up to 30 km/s in a rarefied air. Acceleration of thin CH films of 1 to 50-μm thickness was investigated both in a free-expansion and plasma-confined regimes with the highest achieved velocities up to 4 km/s. The SW damping law in a free space independently on laser intensity and air pressure could be approximated by a power law x ~ tn with a power indexes n1 = 0.85 - 0.95 at the initial stage and n2 = 0.5 - 0.6 later, when a distance of the SW front from a target became comparable with a size of the irradiated spot. Instability growth at contact interfaces between ablative plasma and accelerated film, as well as between plasma and compressed air were observed and compared for various initial irradiation non-uniformities. They were introduced by a grid, which was set in front of the film target.
Hot electrons may significantly influence interaction of ultra short laser pulses with solids. Accurate consideration of resonant absorption of laser energy and hot electrons generation at a critical surface was achieved through the developed physical and mathematical models. 2D ray tracing algorithm has been developed to simulate laser beam refraction and Bremsstrahlung absorption with allowance for non-linear influence of a strong electromagnetic field. Hot electrons transport was considered as a straight-line flows weakening by a friction force calculated in the approximation of the average state of ionization. Developed models were coupled with 2D Lagrangian gas dynamic code "ATLANT" that takes into account non-linear heat transport. The developed program has been applied to simulate irradiations of Al foils by picosecond laser double pulses. Hot electrons transport and heating resulted in thin foil explosions. The transition from exploding foil regime to the ablative one with foil thickening has been simulated and analyzed at various values of laser light intensity. In second series of calculations we have modeled the interaction of nanosecond iodine laser with two-layered target.
United algorithm LATRANT for a simulation of a radiative gas flows in 2D cylindrical geometry has been developed on the basis of the gas-dynamic code ATLANT and the program for the radiation transport calculation LATRA. The developed program takes into account the radiation transport in the multi-group approximation and the gas dynamics within the framework of the improved Lagrangian method. Two modifications of LATRANT code for spherical and plane problems in r-z geometry respectively have been developed. A quasi-one-dimensional simulation of the heating and compression of a two-layer spherical target irradiated by isotropic X-rays has been carried out to demonstrate qualitative difference between LATRANT and three-temperature approximation results. Essentially 2D simulation of compression of the same target by the angular-nonhomogeneous radiation has revealed fuel preheating and radiative symmetrization of the inner shell compression, which are typical for the indirect compression schemes. Transition of infrared radiation to X-rays has been observed in 2D simulation of plain Al foil acceleration by Nd laser pulse.
Hot electrons may significantly influence interaction of ultra short laser pulses with solids. Accurate consideration of resonant absorption of laser energy and hot electrons generation at a critical surface was achieved through the developed physical and mathematical models. 2D ray tracing algorithm has been developed to simulate laser beam refraction and Bremsstrahlung absorption with allowance for non-linear influence of a strong electromagnetic field. Hot electrons transport was considered as a straight-line flows weakening by a friction force calculated in the approximation of the average state of ionization. Developed models were coupled with 2D Lagrangian gas dynamic code "ATLANT" that takes into account non-linear heat transport. The developed program has been applied to simulate irradiations of Al foils by picosecond laser double pulses. Hot electrons transport and heating resulted in thin foil explosions. The transition from exploding foil regime to the ablative one with foil thickening has been simulated and analyzed at various values of laser light intensity.
The 2D codes "ATLANT-C" (Lagrangian coordiantes) and "NUTCY" (Euler coordinates) have been used for the modeling of experiments performed at laser installations "GARPUN" (KrF-laser with pulse energy about 100 J and duration of 100 ns) and "PICO" (Nd-laser with pulse energy of 30 J and duration of 3 ns). Both laser installations are located at Lebedev Physical Institute, Moscow, Russia.
We have suggested a concept of laser-driven shock tube (LST) for generation of hypersonic shock waves (SW) in gases and compression waves in liquids. This novel laboratory technique might be applied to the studies of various fundamental hydrodynamic phenomena such as development of hydrodynamic instabilities at contact interfaces between different liquids and gases accelerated by shock waves, hypersonic gas flow around bodies, effects of strong shock wave refraction and cumulation in time scale of several microseconds and space scale of ten millimeters. These problems are of great importance in Inertial Confinement Fusion, comsology, astrophysics, and aerospace engineering. In this paper we present both numerical simulations and first experimental results to verify the laser-driven shock tube concept for studying of strong SW generation in the air.
We introduce the model of growth of the small perturbations on the boundaries of an incompressible fluid shell with accounting for the ablation effect. Basing on this model we consider the following problems: (i) Behavior of perturbations at 'thin' and 'thick' shell; (ii) Development of perturbations at the outer boundary for different regimes of outer pressure; (iii) Development of RTI at a corresponding unstable boundary in cases of the shell compression and expansion, for the work of external forces is equal for both cases; (iv) The effect of ablation on the RTI development process.
The design of a miniature laser shock tube for the study of a wide range of hydrodynamic phenomena in liquids at pressures greater than 10 kbar and supersonic flows in gases with large Mach numbers is discussed.
The impact of a laser prepulse on the expansion of plasma heated by a strongly non-0uniform main laser pulse is studied experimentally by a side-on shadowgraphy. Our numerical simulations reproduced well the qualitative features of the observed plasma expansion and the smoothing effect of the prepulse .The numerical model identifies sharp contact boundaries between the colliding plasma streams, that are only gradually relaxed with the advancing time.
The construction of powerful multi-beam Nd-laser facilities with pulse energy of 1-2 MJ for the demonstration of 'ignition' in thermonuclear targets is in progress in USA and France. The projects of such facility are also developed in Russia. Such facility is supposed to be built for a few stages. At the intermediate stage it will operate only with a few beams and high symmetry of the spherical target irradiation might not be obtained. The report present the results of 2D beam powerful Nd-laser with energy 100 kJ and pulse duration of 3 ns. On the basis of our simulation we obtained that in case of appropriate profile of laser irradiation of the targets with adjusted thickness relief one can get relatively high neutron yield at the given laser energy.
The compression and the thermonuclear energy release in conic targets with additional laser heating are calculated. It is shown that irradiation of conic targets with a compound KrF laser pulse with energy 300-400 kJ can provide a neutron yield of >= 1016 per shot and, accordingly, target energy gain coefficients above 0.1. As a result, it is possible to create the high repetition rate thermonuclear neutron source with yield 1016-1017 per shot on the basis of UV-laser.
This paper describes a performance of 100 J-class KrF laser system `GARPUN' intended for target irradiation experiments by 100 ns pulses. A controllable space-time distribution in a focal spot reaching 5*1012 W/cm2 produced megabar ablation pressure, which irradiated conical shock wave in solids. It propagated in a quasi-steady manner together with an ablation front that resulted in anomalous high penetration rate of laser radiation throughout the matter. Long-time sample loading together with strong tangential shear flow of compressed layers produced favorable conditions for pressure-induced transformation of the pyrolytic graphite into a diamond-like phase by martensitic mechanism.
The results of the experiments at the installation 'PICO' with thin foils heating by laser radiation pulses of nanosecond duration are reported. The Al foils with thickness in the range from 3 (mu) up to 40 (mu) where used as a targets. The flux density was varied from 1013 W/cm2 to 1014 W/cm2. The sharp dependence of the portion of laser energy passed through the target on foil thickness was observed. This phenomena was accompanied by relatively small decrease of the passed radiation pulse duration. The anomalously high speed burning through of thin foil was observed in these experiments and the conclusion on possible mechanism of this phenomena has been done on the base of comparison of experimental data with theoretical calculations. The observed phenomena can be interpreted on the base of conjecture about the local burning through of a target in the small areas at the target surface with much more values of flux density than average one and following laser radiation self-focusing and formation of 'hot spots.'
It is necessary to reach large gains in laser target (G) to get useful energy in a fusion reactor. Using the fission blanket in a reactor allows one to increase the energy gain. The first Russian projects of laser fission-fusion reactor were studied. It was suggested to use a two-cascade hybrid reactor scheme, allowing one to reach gain in fission blanket more than 1000. As a result it will be possible to use laser target with G on the order of 0.1 - 1 in such type of a reactor. It was suggested to target the design for the 'ignition' experiment at the laser energy 0.1 - 0.3 MJ. It was a high aspect ratio cryogenic direct driven target. But it is necessary to use a lot of laser beams around the target to provide spherical symmetry of laser irradiation. Large surface 'will be lost' for fission blanket. In addition the laser pulse should have a sharp time profile. It is necessary to use about five-to-ten times more laser energy to get 'ignition' in indirect driven target. The new target design, named 'Greenhouse target,' was suggested. It is a very important problem to compensate the negative influence of holes on compression symmetry. It was suggested to use the target with 'relief' to compensate this effect. This target design is studied now. We are studying four alterative approaches to the laser target design for hybrid reactor: (1) the target with inner laser energy input; (2) high aspect ratio target for laser with pulse duration about 100 ns; (3) conic target; (4) exploding pusher spherical or conic targets for carbon-dioxide laser. The numerical simulations were made by using Lagrange code 'ATLANT'.
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