Investigation of light absorption, energy transfer, and plasma dynamic processes in laser-irradiated targets of low average density

Abstract

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.

Time-dependent measurement of high-power laser light reflection by low-Z foam plasma

“…Therefore, a possible correction to the transmitted light in the MULTI-FM simulations cannot exceed about 10%. The simulated transmittance of 1%-4% corrected by a factor about 0.9, according to the considerations just explained, together with the experimental value of the total reflected and scattered light of 10%-12%, leads to an absorption efficiency of about 85%-90%, in very good agreement with the literature [21,22] and much higher than that for a solid target of the same chemical composition, which we measured to be around 70%.…”

“…Considering that in our experiments we can estimate that the focusing lens collects approximately 80% of the reflected light, we obtain a total reflectivity of around 0.12, in agreement with the results of Refs. [21,22]. This value is about three times lower than that of the solid target, for which, at the same way, the total reflectivity results to be approximately 0.32.…”

“…The homogenization time (Equation ( 3)) is confirmed by the experiments in Refs. [6,7,14,18,22,24] where the speed of the absorbed laser energy transfer in a porous substance of supercritical and subcritical density was measured.…”

“…This is the reason for the higher absorption efficiency of laser radiation in the plasma of a porous substance compared with the plasma of a solid target. Experimentally, the absorption efficiency of foams has been measured to be up to 90% for the first harmonic of a nanosecond Nd-laser radiation [6,7,21,22] . The plasma homogenization process determines the features of energy transfer in the foam target.…”

Aerogel foil plasma: Forward scattering, back scattering, and transmission of laser radiation

“…It is proposed to use the method of a dynamic plasma phase plate (Voronich et al, 2001) for smoothing spatial intensity distribution of heating radiation on the target. The optical transparency of a plasma layer is an essential method of a dynamic plasma phase plate, which leads to the necessity of using the low density volume-structured media (Borisenko et al, 1994(Borisenko et al, , 2008Khalenkov et al, 2006;Moreau et al, 2009;Yu et al, 2009b) Volume-structured materials are considered as different functional elements in the inertial confinement fusion (ICF) targets (Dunne et al, 1995;Gus'kov & Rosanov, 1997;Bugrov et al, 1997Bugrov et al, , 1999aBugrov et al, , 1999bNazarov et al, 1999;Cook et al, 2008;Nobile et al, 2006). First of all, smoothing of a heating inhomogeneity and production of a steady compression of the ICF targets prove to be possible applications of those materials.…”