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New results obtained in experiments on laser irradiation (I = 5 × 1013 W/cm2, λ = 1.054 µm) of low-density (2–10 mg/cm3) porous materials (agar, triacetate cellulose, and foam polysterene) are presented and discussed from the standpoint of optimum porous material utilization in target designs for inertial confinement fusion. The influence of low-density material microstructure of irradiated samples on the absorption of laser radiation and the energy transfer processes was investigated using X-ray and optical diagnostic methods with high temporal and spatial resolution.
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 109–1011
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.
The interaction of powerful laser and X-ray pulses
with planar low average density (0.5–10 mg/cm3)
porous agar-agar targets was experimentally studied. At
a laser power density of ∼5 × 1013
W/cm2 (λ = 1.054 μm) the laser light
absorption and following energy transfer processes, as well
as dynamics of produced plasma were investigated in detail
with a variety of optical and X-ray diagnostic methods. Volume
absorption is shown to occur in experiments with laser-irradiated
agar targets. An extended laser energy deposition region filled
with hot (0.8–1 keV) plasma is formed inside a porous
target. The laser light absorption efficiency is as high as ∼80%.
The emission of 2ω0 and 3ω0/2 harmonics
from laser-produced plasma is observed over the time of the laser pulse
even with agar targets of 0.5 mg/cm3 average density.
Characteristics of energy transfer in low-density porous media are
measured in experiments on illumination of agar targets by laser
pulses or X rays emitted by a thin Cu converter. The hydrodynamic
mechanism is responsible for the energy transfer in laser-illuminated
porous targets and the radiative energy transfer seems to be dominant
in the case of X-ray irradiation. The experimental data are in
reasonable agreement with predictions of a developed theoretical model
describing the hot plasma layer formation and the two-stage homogenization
process within the illuminated porous targets.
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 1013-1014 W/cm2. 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.
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