N. V. Zmitrenko

Theoretical and experimental studies of the radiative properties of hot dense matter for optimizing soft X-ray sources

Theoretical and experimental studies of radiative properties of hot dense plasmas that are used as soft X-ray sources have been carried out depending on the plasma composition. Important features of the theoretical model, which can be used for complex materials, are discussed. An optimizing procedure that can determine an effective complex material to produce optically thick plasma by laser interaction with a thick solid target is applied. The efficiency of the resulting material is compared with the efficiency of other composite materials that have previously been evaluated theoretically. It is shown that the optimizing procedure does, in practice, find higher radiation efficiency materials than have been found by previous authors. Similar theoretical research is performed for the optically thin plasma produced from exploding wires. Theoretical estimations of radiative efficiency are compared with experimental data that are obtained from measurements of X-pinch radiation energy yield using two exploding wire materials, NiCr and Alloy 188. It is shown that theoretical calculations agree well with the experimental data.

Nonequilibrium laser-produced plasma of volume-structured media and ICF applications

This paper is devoted to the investigation of powerful laser pulse interaction with regularly and statistically volume-structured media with near critical average density and properties of laser-produced plasma of such a media. The results of the latest experiments on laser pulse interaction with plane foam targets performed on Nd-laser facilities ABC in the ENEA-EURATOM Association (Frascati, Italy) and MISHEN in the Troitsk Institute of Innovation Thermonuclear Investigations (TRINITI, Troitsk Russia), and J-laser ISKRA-4 in the Russian Federal Nuclear Center, All-Russian Scientific Research Institute of Experimental Physics (RFNC-VNIIEF, Sarov, Russia) are presented and analyzed. High efficiency of the internal volume absorption of laser radiation in the foams of supercritical density was observed, and the dynamics of absorbing region formation and velocity of energy transfer process versus the parameters of porous matter are found. Some inertial confinement fusion (ICF) applications based on nonequilibrium properties of laser-produced plasma of a foam and regularly structured media such as the powerful neutron source with yield of 10 9 -10 11 DT-neutrons per 1 J of laser energy, laser-produced X-ray generation in high temperature supercritical plasma, and the compact ICF target absorbers providing effective smoothing and ablation are proposed.

Impact-driven shock waves and thermonuclear neutron generation

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.

A Method for Calculating the Effective Charge of Ions Decelerated in a Hot Dense Plasma

A method for calculating the effective charge of fast ions decelerated in a hot dense plasma is proposed. The method is based on the known experimental dependence of the effective charge of an ion decelerated in cold matter on its velocity. The ion velocity in this dependence is replaced with the velocity of an ion relative to plasma electrons, averaged over the Fermi-Dirac distribution. Using results of numerical calculations performed in a wide range of plasma parameters (from a Maxwellian plasma to a fully degenerate one), a scale-invariant representation of the effective charge of a decelerating ion as a function of its initial velocity and the plasma temperature and density is obtained. An analytical formula fitting the calculated results to within 5% is derived. The obtained dependences of the effective charge are incorporated in the model describing deceleration of fast ions in plasma. Using this model, the stopping powers of krypton and lead ions in a relatively cold rarefied gas-discharge plasma and hot ICF plasma are calculated. The results of calculations are shown to agree satisfactorily with available experimental data.

Fast ignition upon the implosion of a thin shell onto a precompressed deuterium-tritium ball

Fast ignition of a precompressed inertial confinement fusion (ICF) target by a hydrodynamic material flux is investigated. A model system of hydrodynamic objects consisting of a central deuterium-tritium (DT) ball and a concentric two-layer shell separated by a vacuum gap is analyzed. The outer layer of the shell is an ablator, while the inner layer consists of DT ice. The igniting hydrodynamic flux forms as a result of laser-driven acceleration and compression of the shell toward the system center. A series of one-dimensional numerical simulations of the shell implosion, the collision of the shell with the DT ball, and the generation and propagation of thermonuclear burn waves in both parts of the system are performed. Analytic models are developed that describe the implosion of a thin shell onto a central homogeneous ball of arbitrary radius and density and the initiation and propagation of a thermonuclear burn wave induced by such an implosion. Application of the solution of a model problem to analyzing the implosion of a segment of a spherical shell in a conical channel indicates the possibility of fast ignition of a spherical ICF target from a conical target driven by a laser pulse with an energy of 500–700 kJ.

Analysis of direct-drive capsule compression experiments on the Iskra-5 laser facility

We have analyzed and numerically simulated our experiments on the compression of DT-gas-filled glass capsules under irradiation by a small number of beams on the Iskra-5 facility (12 beams) at the second harmonic of an iodine laser (λ = 0.66 μm) for a laser pulse energy of 2 kJ and duration of 0.5 ns in the case of asymmetric irradiation and compression. Our simulations include the construction of a target illumination map and a histogram of the target surface illumination distribution; 1D capsule compression simulations based on the DIANA code corresponding to various target surface regions; and 2D compression simulations based on the NUTCY code corresponding to the illumination conditions. We have succeeded in reproducing the shape of the compressed region at the time of maximum compression and the reduction in neutron yield (compared to the 1D simulations) to the experimentally observed values. For the Iskra-5 conditions, we have considered targets that can provide a more symmetric compression and a higher neutron yield.

Laser-generated weak shock wave propagation dynamics in the solids

The processes of the weak shock wave generation and propagation in the solid targets are studied by analytical and numerical methods. The impact of the laser pulse with wavelength (lambda) equals 1.06 micrometers and the pulse intensity I equals 1010-1012W/cm2 was considered as a wave source. The resulting wave with pressure about 1-100kBar is studied in a solid medium 0.5-1 mm thick. The results of modeling compared with experimental data shows correctness of hydrodynamical approximations for a given problem.

Thermonuclear targets for direct-drive ignition by a megajoule laser pulse

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.

Fast ignition when heating the central part of an inertial confinement fusion target by an ion beam

We investigate the ignition and burning of a precompressed laser fusion target when it is rapidly heated by an ion beam with the formation of a temperature peak in the central part of the target. We present the results of our comprehensive numerical simulations of the problem that include the following components: (1) the target compression under the action of a profiled laser pulse, (2) the heating of the compressed target with spatially nonuniform density and temperature distributions by a beam of high-energy ions, and (3) the burning of the target with the initial spatial density distribution formed at the instant of maximum target compression and the initial spatial temperature distribution formed as a result of the compressed-target heating by an ion beam. The dependences of the threshold energies of the igniting ion beam and the thermonuclear gain on the width of the Gaussian beam ion energy spectrum have been established. The peculiarities of fast ignition by an ion beam related to the spatial distribution of parameters for the target precompressed by a laser pulse are discussed.

Ejection of heavy elements from the stellar core to the periphery of the cloud of ejecta during a supernova explosion: A possible model of the processes

The possibility of simulating the processes during supernova explosions in laboratory conditions using powerful lasers (laboratory astrophysics) is investigated. The Chandra observations of ejecta in the Cassiopeia A supernova remnant are analyzed. Based on the DIANA and NUTCY numerical codes, we have performed 1D and 2D hydrodynamic simulations of the ejecta expansion dynamics for a supernova with a mass of ∼5–15 solar masses within several hundred seconds after its explosion, including an initial asymmetry. We propose a model for the explosion and expansion of ejecta that illustrates strong inhomogeneities in the distribution of material to the extent that the Fe, Si, and S material from the stellar center turns out to be ejected to the periphery, the “star turns inside out,” in agreement with observations. Based on hydrodynamic similarity criteria, we consider possible supernova-simulating laser targets that will allow one to reproduce the physical processes that take place during the explosion of an astrophysical object, such as the shock propagation through the material, the growth of hydrodynamic instabilities at the boundaries of envelopes with different densities, etc.