[en] We consider the generation of beams of highly ionised atoms in solid targets irradiated with CO₂-laser pulses. We present experimental results on generation of Mg and Pb ions from laser plasma at a radiation flux density q ≈ 10¹⁴ W cm^-2. We have developed a theoretical model describing the plasma heating by CO₂-laser radiation at a high pulse intensity on the target, taking into account the ponderomotive forces affecting the behaviour of the interaction of light with the plasma. It is shown that in the case of resonance absorption of laser radiation by the plasma, the efficiency of generation of highly ionised atoms of the target substance is higher than the efficiency of generation in the case of classical absorption. The results of the numerical calculation by the developed model are in good agreement with the experiment.

[en] We report on acceleration of thin foils by the pressure of foam matter heated by a 400 ps pulse iodine laser system PALS. The laser beam interaction with low density foam is investigated here, both experimentally and theoretically. Light element foams as well as foams containing addition of medium Z element (Cl, Cu, Sn) are used. Comparison of foams with various pore size, structure and average density is carried out. Laser pulses of first and third harmonics in range from 100 J to 600 J are focused normally on the foam layer. Laser intensities in range 10''13 up to 10''16 W/cm''2 are achieved by varying the focal spot size. X-ray streak camera is used for evaluation of the speed of energy transfer through the porous foam material. Fast penetration of laser radiation through subcritical foams with small pores is revealed. Shock wave breakthrough onto the rear side of the target is detected via optical self-emission by optical streak camera. The dependence of the shock wave delay on the rear side on foam and laser parameters is observed. Three-frame optical shadowgraphy is employed simultaneously for measurement of the acceleration of thin foil at the target's rear side. Experimental results are compared with our two-dimensional hydrodynamics simulations and with and approximate analytical model. (Author)

[en] Investigations of efficiency of macroparticle acceleration and crater creation processes for the two wavelengths of the PALS (Prague Asterix Laser System) facility laser beam: λ₁=1.315 μm and λ₃=0.438 μm, and two types of targets made of Al: single massive target and double target consisting of a foil (thickness of 6 and 11 μm) placed before the massive target at the distance of 200-500 μm are presented. Targets were irradiated by the iodine laser beam: E_L=120-240 J, the focal spot diameter of 250 μm, and the pulse duration of 0.4 ns. Velocities of the accelerated macroparticles as well as electron density distributions of plasma stream were determined by means of a 3-frame interferometry. Shape and volume of craters were obtained employing crater replica technology and microscopy measurement. Experimental results were analyzed and interpreted by means of two-dimensional theoretical and numerical simulations. Energy transfer as well as two- dimensional shock wave generation and crater formation at the collision of laser-driven macroparticle with massive target have been described. The values of laser energy absorption coefficient, ablation loading efficiency and efficiency of energy transfer at the laser-driven macroparticle impact have been obtained at the different wavelength of laser radiation by crater volume measurement data. (author)

[en] The possibility of efficient and symmetric compression of a target with a low-density structured absorber by a few laser beams is considered. An equation of state is proposed for a porous medium, which takes into account the special features of the absorption of high-power nanosecond laser pulses. The open version of this target is shown to allow the use of ordinary Gaussian beams, requiring no special profiling of the absorber surface. The conditions are defined under which such targets can be compressed efficiently by only two laser beams (or beam clusters). Simulations show that for a 2.1-MJ laser pulse, a seven-fold gain for the target under study is achieved. (special issue devoted to the 80th anniversary of academician n g basov's birth)

[en] The class of direct-drive targets for the laser pulse of a megajoule scale is considered in the work. A distinctive feature of the design of these targets is a relatively low aspect ratio (outer DT-shell radius/shell thickness, A ∼ 10) to provide greater compression stability. The irradiation of the target surface is carried out by the shaped laser pulse on 2ω of Nd-laser. The influence of laser energy absorption inhomogeneity on the parameters of compression and burning of thermonuclear fuel on the final stage of implosion is studied in the work based on 1D and 2D numerical calculations. The results of 2D modeling show that in the case of target's offset from the point of beams crossing significantly greater reduction in the neutron yield is observed than in the case when only irregularities caused by the geometry of irradiation by the finite number of beams are taken into account. (paper)

[en] Development of direct drive target schemes for the megajoule facility is a topical problem of up-to-date inertial fusion physics. The choice of possible schemes and solutions depends essentially on the irradiation conditions. The installations both running (NIF) and under construction (LMJ) are destined to the 3ω irradiation in PDD (polar direct drive) configuration. The UFL-2M installation that is under construction is based on 2ω irradiation and a symmetrical scheme of direct drive target irradiation. Under these conditions possible schemes for direct drive targets demonstrating the ignition and the achievement of gain G=10÷20 are considered in this report. At the same time, the possibilities are analyzed for the target compression and ignition with a reliability reserve at the conditions that can deviate from the standard ones, and if our understanding of the physics of the processes is not completely adequate to the physics of the real processes. (paper)

[en] The article presents calculation results, which were received for the implosion of the typical cryogenic thermonuclear direct-drive targets that are intended for use at the OMEGA facility, NIF and Russian laser facility. The compression and burning characteristics, which were obtained using various numerical codes of different scientific groups, are compared. The data indicate good agreement between the numerical results. Various sources of target irradiation inhomogeneity and their influence on the implosion parameters are considered. The nominal scales of these disturbances for various facilities are close to each other. The main negative effect on the efficiency of compression and burning is due to the accidental offset of the target from the center of the chamber. (paper)

[en] A theoretical model is proposed for computing simulations of laser radiation interaction with inhomogeneous foam materials doped with heavy elements and undoped materials. The model satisfactorily describes many experiments on the interaction of the first and third harmonics of a 200 J pulsed PALS iodine laser with low-density porous cellulose triacetate targets. The model can be used to analyze experimental data and estimate the reality of experimental results.

[en] Impact-driven shock waves, thermonuclear plasma and neutron yield were investigated. The results of 2D numerical simulations and Gekko/HIPER laser experiments on the collision of a laser-accelerated disk-projectile with a massive target, both containing (CD)_n-material, are discussed. A two-temperature model of the non-equilibrium plasma created by impact-driven shock waves due to the collision of a laser-accelerated planar projectile with a massive target was developed and used for analysis of the numerical and experimental results. The model defines the characteristics of shock waves and plasmas (including their lifetime) as well as neutron yields in both the colliding objects as functions of velocity, density and mass of the projectile-impactor just before collision. The neutron yield generated during the period of laser-driven acceleration of the impactor was also determined. Two effects were discovered that exert a substantial influence on the plasma parameters and neutron yield. The first of them relates to the formation of the pre-impact state of the impactor. It decreases the projectile density due to thermal expansion of its matter through a free boundary during the period of laser-driven acceleration. The other relates to the formation of impact-produced plasma. Predominant heating of the ion component of plasma leads to the existence of a non-equilibrium two-temperature plasma during the period of electron-ion relaxation.

[en] Previous experimental results demonstrated that the plasma pressure decreases with the growing atomic number of the target material. In this context, a question arose if the Al plasma outflow could be collimated using the plastic plasma as a compressor. To solve this problem, an experiment using a plastic target with an Al cylindrical insert was performed. The focal spot diameter substantially larger than that of the insert ensured simultaneous heating both target materials. This experiment proved that a production of Al plasma jets collimated by an action of outer plastic plasma is feasible [Kasperczuk et al., Laser Part. Beams 30, 1 (2012)]. The results of investigations presented here provide additional information on distributions of electron temperature in the outflowing plasma and time and space characteristics of ion emission, both registered at bare and constrained-flow Al targets. The experiment was carried out at the Prague asterix laser system iodine laser facility. The laser provided a 250 ps (full width at half maximum) pulse with the energy of 130 J at the third harmonic frequency (λ₃ = 0.438 μm). A plastic target with an Al cylindrical insert of 400 μm in diameter as well as a bare Al target (for comparison) was used. The focal spot diameter (Φ_L) 1200 μm ensured the lateral pressure effect of the plastic plasma strong enough to guarantee the effective Al plasma compression. The electron temperature measurements have shown that such Al plasma compression is accompanied by the increase of its temperature, dominance of which starts at distance of 0.5 mm from the target surface. Measurements of ion emission characteristics confirm the earlier numerical simulation prediction that in these conditions the plasma expansion geometry is closer to planar. The constrained Al plasma jet is very narrow and its axial velocity is considerably larger than the velocity of freely expanding Al plasma stream. It means that the plastic plasma envelope, besides the Al plasma compression, also strongly accelerates the Al plasma in its axial motion.