Prediction and observation of tin and silver plasmas with index of refraction greater than one in the soft x-ray range

Time-resolved absorption spectroscopic characterization of ultrafast laser-produced plasmas under varying background pressures

Time-resolved tunable laser absorption spectroscopy is used to characterize the physical properties of ultrafast laser-produced plasmas. The plasmas were produced from an Inconel target, with ≤0.4wt% Al, using ∼35fs, ∼800nm, ∼5mJ laser pulses at varying Ar background pressures from 1 to 100 Torr. The absorption spectrum of atomic Al is measured with high spectral and temporal resolution when the probe laser is stepped across the selected Al transition at 394.4 nm. Spectral fitting is used to infer linewidths, kinetic temperature, Al column density, and pressure broadening coefficient. The late time physical properties of plasmas are compared for various pressure levels. Our studies highlight that a significant lower state population exists even at early times of ultrafast laser-produced plasma evolution, and lower state population persistence decreases with increasing ambient pressure. We also show that the fundamental optical properties, such as pressure broadening, can be measured using ultrafast laser-produced plasmas combined with laser absorption spectroscopy.

Average-atom treatment of relaxation time in x-ray Thomson scattering from warm dense matter

The influence of finite relaxation times on Thomson scattering from warm dense plasmas is examined within the framework of the average-atom approximation. Presently most calculations use the collision-free Lindhard dielectric function to evaluate the free-electron contribution to the Thomson cross section. In this work, we use the Mermin dielectric function, which includes relaxation time explicitly. The relaxation time is evaluated by treating the average atom as an impurity in a uniform electron gas and depends critically on the transport cross section. The calculated relaxation rates agree well with values inferred from the Ziman formula for the static conductivity and also with rates inferred from a fit to the frequency-dependent conductivity. Transport cross sections determined by the phase-shift analysis in the average-atom potential are compared with those evaluated in the commonly used Born approximation. The Born approximation converges to the exact cross sections at high energies; however, differences that occur at low energies lead to corresponding differences in relaxation rates. The relative importance of including relaxation time when modeling x-ray Thomson scattering spectra is examined by comparing calculations of the free-electron dynamic structure function for Thomson scattering using Lindhard and Mermin dielectric functions. Applications are given to warm dense Be plasmas, with temperatures ranging from 2 to 32 eV and densities ranging from 2 to 64 g/cc.

Refractive index in warm and hot dense matter

A method to estimate the index of refraction in warm and hot dense matter is proposed. This method combines the Kubo-Greenwood approach, Maxwell equations, and existing codes that calculate photoabsorption and photoemission coefficients in warm and hot dense plasmas. An effective electrical conductivity is calculated from existing opacity codes from which the index of refraction is derived. Illustrations are shown on specific examples.

Extreme ultraviolet interferometry of warm dense matter in laser plasmas

We demonstrate that interferometric probing with extreme ultraviolet (EUV) laser light enables determination of the degree of ionization of the "warm dense matter" produced between the critical and ablation surfaces of laser plasmas. Interferometry has been utilized to measure both transmission and phase information for an EUV laser beam at the photon energy of 58.5 eV, probing longitudinally through laser-irradiated plastic (parylene-N) targets (thickness 350 nm) irradiated by a 300 ps duration pulse of wavelength 438 nm and peak irradiance 10(12) W cm(-2). The transmission of the EUV probe beam provides a measure of the rate of target ablation, as ablated plasma becomes close to transparent when the photon energy is less than the ionization energy of the predominant ion species. We show that refractive indices η below the solid parylene N (η(solid) = 0.946) and expected plasma values are produced in the warm dense plasma created by laser irradiation due to bound-free absorption in C(+).

Investigation on the properties of a laminar grating as a soft x-ray beam splitter

Laminar-type gratings as soft x-ray beam splitters for interferometry are presented. Gold-coated grating beam splitters with 1000 lines/mm are designed for grazing incidence operation at 13.9 nm. They are routinely fabricated using electron beam lithography and ion etching techniques. The laminar grating is measured to have almost equal absolute efficiencies of about 20% in the zeroth and -1st orders, which enables a fringe visibility up to 0.99 in the interferometer. The discrepancy of the grating profiles between the optimized theoretical and the experimental results is analyzed according to the comparison of the optimized simulation results and the measurement realization of the grating efficiencies. By a precise control of the grating profile, the grating efficiency in the -1st order and the fringe visibility could be improved to 25% and 1, respectively.

Low-frequency plasma conductivity in the average-atom approximation

Low-frequency properties of a plasma are examined within the average-atom approximation, which presumes that scattering of a conducting electron on each atom takes place independently of other atoms. The relaxation time tau distinguishes a high-frequency region omegatau>1 , where the single-atom approximation is applicable explicitly, from extreme low frequencies omegatau<1 , where, naively, the single-atom approximation is invalid. A proposed generalization of the formalism, which takes into account many-atom collisions, is found to be accurate in all frequency regions, from omega=0 to omegatau>1 , reproducing the Ziman formula in the static limit, results based on the Kubo-Greenwood formula for high frequencies and satisfying the conductivity sum rule precisely. The correspondence between physical processes leading to the conventional Ohm's law and the infrared properties of QED is discussed. The suggested average-atom approach to frequency-dependent conductivity is illustrated by numerical calculations for an aluminum plasma in the temperature range 2-10eV .

Dynamics of converging laser-created plasmas in semicylindrical cavities studied using soft x-ray laser interferometry

The evolution of dense aluminum and carbon plasmas produced by laser irradiation of 500-microm -diam semicylindrical targets was studied using soft x-ray laser interferometry. Plasmas created heating the cavity walls with 120-ps -duration optical laser pulses of approximately 1x10;{12}Wcm;{-2} peak intensity were observed to expand and converge on axis to form a localized high-density plasma region. Electron density maps were measured using a 46.9-nm -wavelength tabletop capillary discharge soft x-ray laser probe in combination with an amplitude division interferometer based on diffraction gratings. The measurements show that the plasma density on axis exceeds 1x10;{20}cm;{-3} . The electron density profiles are compared with simulations conducted using the hydrodynamic code HYDRA, which show that the abrupt density increase near the axis is dominantly caused by the convergence of plasma generated at the bottom of the groove during laser irradiation.