[en] A two-stage optical parametric amplifier (OPA) design that provides over  −5  ×  10⁷ gain at 1053 nm is presented. Noise level of the parametric amplifier in the signal wave propagation direction was 2  ×  10⁻⁶ from the signal level. The parasitic parametric oscillation under intense pump wave and its contribution to the OPA output was measured to be less than 8%. (letter)

[en] Parameters of the optical parametric amplifier (OPA), based on two BBO crystals were studied. The OPA was made with the schematic of the extraordinary wave walk off compensation. Efficient amplification of the weak signal ( λ   =  1053 nm) in the field of the strong pumping wave ( λ   =  532 nm) was obtained. The measured value of the amplification was equal to ∼10⁶. The noise level of the parametric amplifier was less than 10⁻³ from the signal level. (paper)

[en] We present the results of preliminary experiments at laser facilities in which the processes of the undeniable destruction of stony asteroids (chondrites) in space by nuclear explosions on the asteroid surface are simulated based on the principle of physical similarity. We present the results of comparative gasdynamic computations of a model nuclear explosion on the surface of a large asteroid and computations of the impact of a laser pulse on a miniature asteroid simulator confirming the similarity of the key processes in the fullscale and model cases. The technology of fabricating miniature mockups with mechanical properties close to those of stony asteroids is described. For mini-mockups 4–10 mm in size differing by the shape and impact conditions, we have made an experimental estimate of the energy threshold for the undeniable destruction of a mockup and investigated the parameters of its fragmentation at a laser energy up to 500 J. The results obtained confirm the possibility of an experimental determination of the criteria for the destruction of asteroids of various types by a nuclear explosion in laser experiments. We show that the undeniable destruction of a large asteroid is possible at attainable nuclear explosion energies on its surface.

[en] The paper is devoted to the numerical and theoretical study of compression and burning of a thermonuclear target under the conditions of inhomogeneous heating due to direct irradiation with a multi-beam megajoule laser facility. The two-dimensional distribution of absorbed laser energy has been numerically calculated taking into account refraction and various known factors of the violation of irradiation homogeneity. The limits of the violation of irradiation uniformity caused by these factors, which are acceptable for spark ignition of the target irradiated with 192 laser beams of the modern megajoule facilities, are determined. The target offset relative to the aiming point of the laser beams should not exceed 2% of the target radius, mispointing of the beams (5% of the target radius), mistiming of the laser pulses (3% of the pulse duration) and energy imbalance among the laser clusters (12%). These values exceed the permissible deviations for the laser facility being developed within the frameworks of the project. (paper)

[en] We have studied the dependence of the compression and burning of a spherical direct-drive fusion target on the nonuniformity of its heating caused by the asynchronous arrival of laser beams under conditions of irradiation by a modern laser system with a total energy of 2 MJ intended for the fuel ignition and fusion energy evolution equal to the absorbed laser energy. The investigation is performed by numerical simulation based on 2D hydrodynamic codes. It is established that the limiting permissible spread of the moments of laser pulse action on the target for ignition significantly exceeds the level that can be ensured using modern methods of controlled temporal synchronization of laser beams.

[en] Central ignition of a thin two-layer-shell fusion target that is directly driven by a 2-MJ profiled pulse of Nd laser second-harmonic radiation has been studied. The parameters of the target were selected so as to provide effective acceleration of the shell toward the center, which was sufficient for the onset of ignition under conditions of increased hydrodynamic stability of the ablator acceleration and compression. The aspect ratio of the inner deuterium-tritium layer of the shell does not exceed 15, provided that a major part (above 75%) of the outer layer (plastic ablator) is evaporated by the instant of maximum compression. The investigation is based on two series of numerical calculations that were performed using one-dimensional (1D) hydrodynamic codes. The first 1D code was used to calculate the absorption of the profiled laser-radiation pulse (including calculation of the total absorption coefficient with allowance for the inverse bremsstrahlung and resonance mechanisms) and the spatial distribution of target heating for a real geometry of irradiation using 192 laser beams in a scheme of focusing with a cubo-octahedral symmetry. The second 1D code was used for simulating the total cycle of target evolution under the action of absorbed laser radiation and for determining the thermonuclear gain that was achieved with a given target

[en] Direct-drive fusion targets are considered at present as an alternative to targets of indirect compression at a laser energy level of about 2 MJ. In this approach, the symmetry of compression and ignition of thermonuclear fuel play the major role. We report on the results of theoretical investigation of compression and burning of spherical direct-drive targets in the conditions of spatial nonuniformity of heating associated with a shift of the target from the beam center of focusing and possible laser radiation energy disbalance in the beams. The investigation involves numerous calculations based on a complex of 1D and 2D codes RAPID, SEND (for determining the target illumination and the dynamics of absorption), DIANA, and NUT (1D and multidimensional hydrodynamics of compression and burning of targets). The target under investigation had the form of a two-layer shell (ablator made of inertial material CH and DT ice) filled with DT gas. We have determined the range of admissible variation of compression and combustion parameters of the target depending on the variation of the spatial nonuniformity of its heating by a multibeam laser system. It has been shown that low-mode (long-wavelength) perturbations deteriorate the characteristics of the central region due to less effective conversion of the kinetic energy of the target shell into the internal energy of the center. Local initiation of burning is also observed in off-center regions of the target in the case of substantial asymmetry of irradiation. In this case, burning is not spread over the entire volume of the DT fuel as a rule, which considerably reduces the thermonuclear yield as compared to that in the case of spherical symmetry and central ignition.

[en] The compression and burning of a fusion target ignited by a focused shock wave produced at the action of a time-profiled second harmonic laser pulse of a Nd laser have been calculated and theoretically studied. The main energy features of the shock ignition scheme have been considered. The use of the second harmonic radiation corresponds to a higher energy and a longer laser pulse necessary for ignition by this method compared to the use of the third harmonic radiation. Nevertheless, the method of ignition by the focused shock wave with the second harmonic radiation makes it possible to reach the fusion target gain that is two or three times higher than that at the traditional spark ignition with the laser pulse energy higher than in the former case by a factor of 1.5. The numerical calculations have been performed with one-dimensional hydrodynamic codes.