Time-resolved x-ray imaging of high-power laser-irradiated underdense silica aerogels and agar foams

Abstract

DISCLAIMER AbstractThis paper presents the results of experiments in which a high-power laser was used to irradiate low density (4 -9 mg/cm3) silica aerogel and agar foam targets. The iaser-solid interaction and energy transport through the material were monitored with time-resolved imaging diagnostics, and the data show the production and propagation of an x-ray emission front in the plasma.The emission-front trajectory data are found to be in significant disagreement with detailed simulations, which predict a much more rapid heating of the cold material, and the data suggest that this discrepancy is not explainable by target inhomogeneities. Evidence suggests that energy transport into the cold material may be dominated by thermal conduction; however, no completely satisfactory explanation for the discrepancies is identified, and further experimental and theoretical research is necessary in order to resolve this important problem in laser-plasma interaction physics.

Supersonic propagation of ionization waves in an underdense, laser-produced plasma

“…The authors observe consistency of the front velocity with radiation driven thermal transport and discuss the transition from the electron conduction to radiation transport as the laser intensity increases from 10 14 to 10 17 W/cm 2 . The closest conditions to our work in terms of target density and Iλ 2 (I is the laser intensity and λ is the laser wavelength) are those reported in the work of Koch et al [11]. The targets are foams at much lower densities than the previously mentioned experiments (4-8 mg/cm 3 ) and the heat waves are driven by a 1 ns laser pulse with intensities in the range of 10 14 -10 16 W/cm 2 .…”

“…Such non-local transport effects have been previously observed in similar plasmas characterized by large electron temperature gradients [11][12][13][14]. More sophisticated computational codes that solve Fokker-Planck equations and treat carefully the non-Maxwellian electron distribution function and electron collisions [15][16][17][18] are necessary to fully describe the transport problem and may constitute the subject of a theoretically oriented article.…”

Numerical modeling of a 2D wave of electron thermal conductivity produced by a laser beam in targets of subcritical density

“…This coordinate was determined with respect to the initial position of the irradiated surface of the target. In [5], an attempt at a theoretical description of the experimental results was made on the basis of numerical calculations within the framework of the model problem of laser interaction (with experimental beam parameters) with a homogeneous medium, the chemical composition of which corresponded to the "agar agar" material and the density of which corresponded to the average density of the experimental targets. The 2D (cylindrical) version of the well-known LASNEX code was used to solve the hydrodynamic equations which include the electron thermal conductivity, radiative transport for the proper plasma radiation, and laser-radiation absorption and take into account some other processes.…”

“…In [5], the LASNEX calculations were performed for the initial stage of interaction within the time interval 0 < t < 0.2 ns. Figures 3a and 3b which correspond to the targets with the densities p = 4.10 -3 and 9 9 10 -3 g/cm 3, respectively, compare the experimental data (curve 1) with the results of calculations using various computer codes.…”

“…The first set of calculations was performed for experimental conditions of the Lawrence Livermore National Laboratory facility (Nova Nd-laser) [5]. In these LLNL experiments, the dynamics of energy transport was studied using a plane target made of "agar-agar" porous material of density p = 9 9 10 -3 g/cm 3 and p = 4 9 10 -3 g/cm 3 irradiated by the second harmonic of Nd-laser radiation (A = 0.523 #m).…”