Yu.A. Merkuliev

Regular 3-D networks with clusters for controlled energy transport studies in laser plasma near critical density

Abstract Fabrication methods for low-density fine-structure (cell size < 1 μm) 3-D networks of cellulose triacetate (TAC) are developed. Target densities ranged 4-20 mg/cm3, similar polymer structures were produced both with no load and with high-Z cluster dopant with concentration up to 30%. Foams of varying density down to 0.25 plasma critical density at the third harmonic of iodine laser wavelength are supplied for laser shots. Closed-cell and 3-D network structures are considered and monitored as the means of thermal and radiation control in plasma. In comparative foam-and-foil laser irradiation experiments on PALS (Czech, Prague) laser facility the presently developed TAC targets were used along with earlier reported TMPTA (trimethylol propane triacrilate) and agar foams. Radiation transport and hydrodynamic wave velocities proved to be similar in TAC and TMPTA volume structures both having the form of regular 3-D networks, but differed a lot when TAC was compared to agar foams. Radiation transport during laser pulse in TAC doped with Cu-clusters was faster then in TAC with no dopant, whereas plasma from TAC doped with Cu-clusters cooled down quicker then with no clusters. High-Z cluster dopant is effective tool to control energy transport in underdense plasma.

Plastic Aerogel Targets and Optical Transparency of Undercritical Microheterogeneous Plasma

Abstract The specifications on different low-density structures and high-Z admixtures are compared for target designs used to achieve smoother energy flux from different drivers. The experimentally realized aerogels are studied. Various characterization methods are used taking into account their heterogeneity. The physical processes in such materials under the powerful laser irradiation are discussed, in particular transparency dynamics, plasma homogenization, smoothing capability. The further applications of low-density cellulose triacetate (TAC) in the physical experiments are: possible targets for ns- and for ps-lasers, for EUV sources, EOS targets with low-density layer for pressure increase.

X-RAY TOMOGRAPHY OF GROWING SILICA GEL WITH A DENSITY GRADIENT

Abstract Density gradient laser targets with decreasing density or increasing stepwise density layers were reported in experimental and theoretical papers on astrophysics modeling, equation-of-state (EOS), and shock-wave dynamics studies. The research with targets of smooth density gradient is due. The experiments with gel-catalyst concentration diffusion are discussed for density gradient foam formation. We used multi-image X-ray tomography for measurement of the silica gel layer density gradient in the process of its growth. Gel with a density gradient growing from a flat boundary between a gel-forming solution and a catalyst solution has been investigated through a set of three-dimensional frames of an X-ray computer microtomograph. Laser targets require high (>1 g·cm-3/cm) density gradients of the spatial profile for EOS experiments. The first targets from silica aerogel with a density gradient are demonstrated. Yet these targets perform less density gradient (<0.1 g·cm-3/cm) than is required for pressure multiplication in EOS targets.

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

Experimental results obtained with “Kanal-2” facility under the study of powerful laser pulse interaction with the low density microstructure media are presented and discussed in this paper. Forward scattering, back scattering, and transmission of laser radiation by aerogel foil plasma have been investigated. The temporal, spectral, and energy characteristics of both the radiation scattering in the direction of heating radiation beam and the back scattering radiation were studied; the directional diagrams of forward and back scattering radiation were obtained for ω 0 and 2ω 0 frequencies. Analysis of intensity redistribution on the heating beam cross-section after passing through a polymer microstructure target was carried out.

X-ray tomography characterization of density gradient aerogel in laser targets

The low-density solid laser target characterization studies begun with the SkyScan 1074 computer microtomograph (CMT) [1, 2] are now continued with higher resolution of SkyScan 1174. The research is particularly focused on the possibility to obtain, control and measure precisely the gradient density polymers for laser target production. Repeatability of the samples and possibility to obtain stable gradients are analysed. The measurements were performed on the mm-scale divinyl benzene (DVB) rods.

Microheterogeneous plasma from polymer aerogel and foam at powerful laser interaction

New results obtained in experiments on laser irradiation (I = 5×1013 W/cm2, λ = 1.054 micrometer) of low-density (10 mg/cm3) fibrous materials (agar and triacetate cellulose) are presented and discussed. The effect of low-density material microstructure of irradiated samples on physical processes in high-temperature plasma was investigated using a variety of plasma diagnostic methods.

From microheterogeneous targets to microheterogeneous plasma—A way to long‐evolution plasma in laser shot

Microheterogeneous targets for plasma investigation are discussed. Microheterogeneity is obtained in the form of metal clusters dispersed inside solid material or foam as well as in the form of metallized aerogel. Microheterogeneous plasma is shown to be initiated by such targets and its behavior is analyzed. The only way to observe the long‐evolution plasma in a short laser experiment is to conserve a certain stage of plasma evolution in the target structure. Target structuring provides: 1) phase separated multi‐ion mixtures in highly non‐ideal plasmas, 2) extended corona (without any prepulse), 3) obscure stability through microinstabilities in the non‐linear growth regime. For ICF applications, underdence microheterogeneous plasma is equalizing the laser irradiation energy over the target‐in‐flight surface.

Novel Technologies in Microfusion

The developed method of foaming for spherical shells made from metastable hydrides such as BeD2 or ND3BD3 is discussed. The importance of low‐density state of meta‐stable hydrides due to the higher energy transfer rates is discussed. The properties of fuel — materials with high content of hydrogen isotopes, which have substantial importance for large fusion target fabrication technology and neutron generation in strong e.m. fields are analyzed. New laser targets and their characteristics are demonstrated.

Intensive (up to 1015 W/cm2) Laser Light Absorption and Energy Transfer in Subcritical Media with or Without High‐Z Dopants

Fabrication methods for low‐density fine‐structure (cell size <1 μm) 3‐D networks of cellulose triacetate (TAC) are developed. Target densities ranged 4–20 mg/cc, similar polymer structures were produced both with no load and with high‐Z cluster dopant with concentration up to 30%. Foams of varying density down to 0.25 plasma critical density at the third harmonic of iodine laser wavelength are supplied for laser shots. Experiments with underdense foam targets with and without clusters irradiated on the PALS laser facility are analyzed preliminary, showing strong influence of target structure on process of laser light absorption. Heat and radiation transport in such targets are considered.