V. V. Gavrilov

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.

Investigation of energy transfer in plane laser-irradiated targets with high X-ray conversion efficiency

Results of experimental and computational investigations devoted to energy transfer mechanisms and X-ray conversion efficiency in laser-produced plasma are presented and discussed. The layers of different thicknesses and diameters deposited on the plane mylar substrate were irradiated by the focused beam of Nd:glass laser. Spectrally, temporally, and spatially resolved measurements of soft X-ray emission have been carried out at power densities of 10 13 -10 14 W/cm 2 . The conditions of “re-emission” zone formation have been established. Radiative heat conductivity is shown to be the important energy transfer mechanism in the experimental conditions under investigation.

Diagnostics for studies of absorption and energy transfer process features in low-density porous targets irradiated with powerful laser pulses

A diagnostic methodology for studies of powerful laser pulse interaction with a porous material of low average density in conditions of interest for advanced inertial confinement fusion targets is discussed. These diagnostics were used in experiments with a powerful 1 μm Nd:laser beam incident upon “agar-agar” targets (C14H18O7)n of different average density from 1×10−3 to 5×10−3 g/cm3 and thickness from 100 to 1000 μm with and without thin aluminium layers (0.5–6 μm) on the front and rear surfaces. Intensity at the focal spot area 100 μm in diameter ranged from 1013 to 1014 W/cm2. It is shown that it is possible to estimate the longitudinal and transverse dimensions of the laser–plasma interaction region and to obtain the plasma dynamics data by recording: the visible and IR radiation at the rear side of illuminated targets with streak camera; x-ray images with filtered pin holes; and multiframe shlieren- and shadowgraphy snapshots (0.3 ns laser pulses at 0.53 μm wavelength have been used).

X-ray diagnostics of plasma generated during collisions of plasma flows

Diagnostics of the high-temperature plasma formed during plasma flow collisions by comparing observable intensities of various x-ray spectral lines to accurate kinetic calculation results is discussed. Kinetic calculations have shown that using the resonance line intensity ratio of He-like Ne IX and 3p–2s Li-like Ne VIII is the most convenient way to measure neon-containing plasma temperatures. Experimental research results of the pulse soft (0.1–1 keV) x-ray source with a total energy up to 50 kJ are shown. The radiation pulse is generated during a head-on collision of two low-temperature plasma flows immersed in a longitudinal magnetic field. The plasma flows with the velocities up to 4 × 107 cm/s and total energy up to 200 kJ are formed by a coaxial accelerator operating in pulse gas puffing mode. Neon was used as an inflating gas. Electron temperature for a plasma containing Ne ions is about 160–170 eV. Electron density around the collision region is in the range 1016–1017 cm−3.

Plasma Diagnostics in Interaction of Powerful Laser Pulses With Inhomogeneous Low-Density Media

Utilization of low-density porous or foam-like materials in laser-irradiated target designs seems to be a very promising approach to the successful realization of such important research programs as ICF, elaboration of X-ray lasers, laboratory modeling of astrophysical phenomena, and EOS measurements of materials in Mbar pressure range. To give some recommendations on application of low-density media in investigations mentioned above it is necessary to study laser light absorption, plasma formation, energy transport, and plasma homogenization in porous matter. These processes are very complicated and have some important features in comparison with the similar processes in the case of laser-irradiated solid-density targets. To have detailed and reliable experimental data we need to use in each experiment the combination of various diagnostics with high spectral, temporal, and spatial resolution. This paper presents the diagnostic complex developed on the “Mishen” facility as well as some obtained experimental results.

Advanced plasma diagnostics for investigation of physical processes in laser-irradiated foam targets (abstract)

Recently the low-density foam-like materials have been found to be very attractive for various applications in high energy-density physics. Irradiation of these materials by powerful laser pulses is a promising approach for plasma formation with parameters being of interest for inertial confinement fusion, x-ray lasers, modeling of astrophysical phenomena, etc. This article is devoted to development and application of diagnostic methods in experiments on irradiation of planar low-density (0.5–10 mg/cm3) porous targets with powerful laser pulses (1013–1014 W/cm2). To obtain reliable information on high-temperature dense plasma formation, plasma dynamics, and energy transfer in the target interior, we used a number of optical and x-ray diagnostics providing high spatial (∼10 μm) and temporal (∼10 ps) resolution. High-speed x-ray imaging, multiframe optical shadowgraphy, and interferometry, as well as scattered laser light spectroscopy at the fundamental frequency and its harmonics were used in each experime...

Physical processes in a laser-greenhouse target: Experimental results, theoretical models, and numerical calculations

The paper is devoted to recent results concerning investigation of physical processes occurring in a “laser greenhouse” target. Results of experimental and theoretical studies of laser-pulse interaction with a low-density absorber of the target, namely, with a porous substance having density close to the plasma critical density, are presented. On the basis of a vast cycle of experiments carried out in a number of laboratories, it is shown that the absorption of the laser radiation in porous media, including those with a density exceeding the critical one by at least a factor of 4 to 6, has a bulk nature and is distributed over the target depth. In particular, the laser-radiation absorption region in a porous substance with density 10−3–10−2 g/cm3 is extended into the target 400–100 μm, respectively. The coefficient of absorption of laser radiation with intensity 1014–1015 W/cm2 in porous substances, including those of the supercritical density, is 70–90%. Experiments have not shown enhanced (compared to a solid-state target) radiation intensity associated with a possible development of parametric instabilities in an extended laser plasma of low-density porous media, as well as noticeable contribution of fast electrons to the energy balance and their effect on the energy transfer. In this paper, theoretical models are developed explaining features of the laser-radiation absorption and energy transfer in porous media. These models are based on the phenomenon of laser-radiation interaction with solid components of a porous substance and plasma production inside pores and cells of the medium. The efficiency of energy conversion in the vicinity of the ignition threshold for the laser-greenhouse target is investigated in the case of an absorber having the above properties. Numerical calculations have shown that a thermonuclear-gain coefficient of 1 to 2 (with respect to the energy absorbed) is attained for a laser-radiation energy of 100 kJ.