V. B. Rozanov

Interaction of a high-power laser beam with low-density porous media

We have experimentally investigated the processes of laser light absorption and energy transfer in porous targets made of “agar-agar” (C14H18O7) with an average density of 1–4 mg/cm3 illuminated by the focused beam of a neodymium laser with an intensity of 1014 W/cm2 within a pulse of duration 2.5 ns. Many important scientific and technical problems, e.g., inertial-confinement thermonuclear fusion, the creation of lasers in the x-ray regime, and the modeling of astrophysical phenomena under laboratory conditions, can be successfully addressed by using low-density porous media as components of such targets. In our experiments with porous targets of variable density and thickness we used optical and x-ray diagnostic methods, which ensured that our measurements were made with high temporal and spatial resolution. We show that a region forms within the porous target consisting of a dense high-temperature plasma which effectively absorbs the laser radiation. Energy is transferred from the absorption region to the surrounding layer of porous material at up to 2×107 cm/s. Experimental data are in good agreement with the predictions of our theoretical model, which takes into account the specific features of absorption of laser radiation in a porous material and is based on representing the energy transfer within the material as a hydrothermal wave.

Absorption and scattering of high-power laser radiation in low-density porous media

We have experimentally investigated the interaction of high-power neodymium laser pulses in the intensity range 1013–1014 W/cm2 with flat low-density (0.5–10 mg/cm3) agar-agar targets under conditions of interest for problems of inertial nuclear fusion. Optical and x-ray methods with high temporal and spatial resolution were used to examine the dependence of absorption and scattering of the incident beam on the initial mean density and thickness of the irradiated samples. We show that when a porous target is irradiated, a bulk absorption layer of high-temperature plasma is produced inside the target whose dimensions are determined by the initial density of the material. The time dependence and spectral composition of the harmonics 2ω0 and 3ω0/2 observed in the plasma-scattered radiation are measured. A theoretical model is developed that describes the interaction of high-power laser pulses with a porous medium. Predictions of the model, based on the hypothesis of two stages of homogenization of the target material—a fast stage (0.1–0.3 ns) and a slow stage (1–3 ns), are in good agreement with the experimental data.

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.

Dynamics of high-temperature plasma formation during laser irradiation of three-dimensionally structured, low-density matter

Results are presented from an experimental investigation of the properties of the plasma produced by the action of a radiation pulse at the second harmonic of a Nd laser, with average intensity ~5·1014 W/cm2 in the focal spot, on flat targets consisting of porous polypropylene (CH)x with an average density of 0.02 g/cm3 (close to the critical plasma density) and with ~50 µm pores. The properties of the laser plasma obtained with porous and continuous targets are substantially different. The main differences are volume absorption of the laser radiation in the porous material and much larger spatial scales of energy transfer. The experimentally measured longitudinal ablation velocity in the porous material was equal to (1.5–3)·107 cm/s, which corresponds to a mass velocity of (3–6)·105 g/cm2· s, and the transverse (with respect to the direction of the laser beam) propagation velocity of the thermal wave was equal to ~(1–2) ·107 cm/s. The spatial dimensions of the plasma plume were ~20–30µm. The plasma was localized in a 200–400µm region inside the target.

On the Possibility of Target Plasma Ignition under the Conditions of Inertial Nuclear Fusion

The thermonuclear gain G for bulk and spark ignitions are calculated using a mathematical simulation of thermonuclear combustion in a DT plasma of laser targets for various parameters of the target plasma and (isobaric and isochoric) ignitors. The critical parameters of ignitors at which an effective nuclear burst occurs with G ∼ 100 are calculated. It is shown that a further increase in the temperature and size of the ignitors virtually does not affect the efficiency of DT fuel burnup. Irrespective of the ignition technique, the value of G can be estimated with the help of a simple asymptotic formula. At the same time, the critical parameters of ignitors are determined to a considerable extent by the mode of ignition and by the target parameters. Spark ignition with an isochoric ignitor corresponding to the fast ignition mode is considered in detail. It is shown that the main critical parameter for optimal isochoric ignitors is their thermal energy liberated upon absorption of an auxiliary ultrashort laser pulse. The critical values of this energy are calculated.

Interaction of nanosecond laser pulses with plastic foams

Some results of an experimental and theoretical study of the interaction of laser pulses with plastic foams are reported. The propagation velocity of a hydrodynamic peturbation which was initiated in foam target under the action of a laser pulse with intensityq≈2·1013 W/cm2 and the velocity distribution function of plasma ions were measured; the preliminary results of time-integrated spectroscopic measurements of an intense red-shifted signal are also reported. A self-consistent model of the foam target’s laser plasma formed in a hydrodynamic mode was derived. The predictions of this model are consistent with experimental results. A model of microprocesses of laser plasma formation in a structured material was also developed. The results of numerical simulations by 1D and 2D computer codes are also reported.

Equations of state for metals (Al, Fe, Cu, Pb), polyethylene, carbon, and boron nitride as applied to problems of dynamical compression

Metals (Al, Fe, Cu, Pb), polyethylene, and other plastic materials with a density of about 1 g/cm3 are commonly used as liners and screens in solving dynamic-compression problems that involve phase transitions. In this paper, the equations of state are presented in the form of formulas, graphs, and tables for the pressurep and energyE as functions of temperatureT and density ρ. These equations have a meaningful theoretical form and are based on the measured initial sound velocityc0, densityρ0, Gruneisen parameter Γ, heat capacitycp, sublimation energyUevp, and the known pressure dependence of the compression modulus ϱK/ϱp. These equations of state are in satisfactory agreement with available experimental data on shock compression. According to the same scheme, the equations of state are derived for carbon and boron nitride. However, in this case, the situation turned out to be much more complicated due to the existence of phase transitions from the hexagonal form into wurtzite and cubic forms. In deriving the equation of state, the equilibrium curve between the graphite-like and diamond phases on the phase diagram was additionally used. As a result of realization of the aforementioned scheme, the equations of state obtained (i.e., formulas, graphs, and tables) are in satisfactory agreement with experimental data.

Powerful thermonuclear neutron source based on laser excitation of hydrothermal dissipation in a volume-structured medium

An efficient method is proposed for generating thermonuclear neutrons by irradiating with a laser pulse a volume-structured material of subcritical density, consisting of a series of thin layers of condensed matter separated by interlayers of low-density matter (or a vacuum gap). The plasma ions are heated up to thermonuclear temperatures much higher than the electron temperature by hydrothermal dissipation of the energy of the laser radiation, as a wave of thermal explosions of the layers propagates along the laser beam axis, followed by collisions of plasma counterflows with conversion of the kinetic energy into thermal energy of ions. Different variants of the targets and experimental conditions are discussed in order to demonstrate the proposed method of neutron generation.

Hydrodynamic instability and target design

The development of hydrodynamic instability (HDI) in laser targets is studied by means of 2D numerical code “ATLANT.” The scaling of HDI development for the condition of “DELFIN-1” laser setup experiments is derived. It is shown that there is the possibility to improve the shell compression condition for the case of long-wave perturbation of laser flux (determined by particular target irradiation geometry) by using the shell with “relief.”