R. V. Stepanov

Laser-driven acceleration of a dense matter up to ‘thermonuclear’ velocities

The results of theoretical studies and numerical simulations of laser-driven acceleration of a flat foil up to ultrahigh velocity of the order of 1000 km s−1, which corresponds to the achievement of thermonuclear temperatures due to kinetic energy transition into thermal energy at an inelastic impact, are reported. The behavior of a foil accelerated to such high velocities, in particular, the distribution of foil density, which defines thermonuclear reaction intensity, has been studied. The calculation results are compared with the results of the experiments performed on the Gekko/HIPER laser, where a laser-driven projectile achieved record-breaking velocity. The laser pulse and foil parameters responsible for acceleration of the projectile up to thermonuclear velocities in a dense state have been determined.

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.

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.

Effect of spatial nonuniformity of heating on compression and burning of a thermonuclear target under direct multibeam irradiation by a megajoule laser pulse

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.

Cassiopeia A: Supernova explosion and expansion simulations under strong asymmetry conditions

We propose a model for the explosion of a supernova and the expansion of its ejecta in the presence of a strong initial asymmetry (at the explosion time) in the central part of the star (core) and a possible smallscale asymmetry in the peripheral regions. The Chandra and NuSTAR observations of ejecta in the Cassiopeia A supernova remnant are analyzed. Based on our 1D and 2D numerical simulations performed using the DIANA and NUTCY codes, we propose a model for the explosion and expansion of ejecta that explains the observed experimental data where the materials initially located in the central region of the star end up on the periphery of the cloud of ejecta.

Study of Possibilities of Simulating the Processes of Asymmetric Explosion and Expansion of Supernovae in a Laser Experiment

Processes of explosion and expansion of laser targets, experiments with which could clarify the reason for the observed asymmetric distribution of matter in a remnant cloud of some supernovae (e.g., the Cassiopeia A supernova), have been simulated. By analyzing criteria of hydrodynamic similarity of conditions characteristic of an astrophysical object and experiment, targets for absorbed laser energies in the range of 1–100 kJ have been proposed. This work continues a series of previous studies of supernovae and the possibility of simulating a number of processes observed at the explosion of supernovae such as the motion of a shock wave in matter, development of hydrodynamic instabilities at interfaces between shells with different densities, and largescale mixing of layers of the central region of a star with elements initially located at the periphery of the remnant cloud under laboratory conditions with high-power lasers. The studies are based on the numerical simulation of the explosion and explosion of targets using one- and two-dimensional hydrodynamic programs.

Fast ignition of asymmetrically compressed targets for inertial confinement fusion

It is shown that fast ignition can ensure the combustion of asymmetrically compressed targets for inertial confinement fusion with an efficiency close to the combustion of one-dimensionally compressed targets. This statement is valid not only for targets specially designed for fast ignition. Fast heating by an external energy source can ensure the ignition of a target designed for spark ignition, but where this ignition does not occur because inhomogeneities are formed in the temperature and density distributions owing to the development of hydrodynamic instabilities. The condition for ignition is the fast heating of the plasma in the combustion initiation region whose size is comparable with the sizes of compression inhomogeneities. Thus, fast ignition not only significantly reduces the ignition energy, but also is possibly a necessary stage in the inertial confinement fusion scheme when the spherically symmetric compression of a target requires very high engineering and financial expenses. The studies are based on the numerical simulation of the compression and combustion of inertial confinement fusion targets with one- and two-dimensional hydrodynamic codes.