Laboratory measurements of resistivity in warm dense plasmas relevant to the microphysics of brown dwarfs.

Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. The inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs.

Spatially resolved analysis of Kα x-ray emission from plasmas induced by a femtosecond weakly relativistic laser pulse at various polarizations.

Spatially resolved K-shell spectroscopy is used here to investigate the interaction of an ultrashort laser pulse (λ=800 nm, τ=40 fs) with a Ti foil under intense irradiation (Iλ(2)=2×10(18)Wµm(2)cm(-2)) and the following fast electron generation and transport into the target. The effect of laser pulse polarization (p, s, and circular) on the Kα yield and line shape is probed. The radial structure of intensity and width of the lines, obtained by a discretized Abel deconvolution algorithm, suggests an annular distribution of both the hot electron propagation into the target and the target temperature. An accurate modeling of Kα line shapes was performed, revealing temperature gradients, going from a few eV up to 15-20 eV, depending on the pulse polarization. Results are discussed in terms of mechanisms of hot electron generation and of their transport through the preplasma in front of the target.

A novel technique for single-shot energy-resolved 2D x-ray imaging of plasmas relevant for the inertial confinement fusion.

A novel x-ray diagnostic of laser-fusion plasmas is described, allowing 2D monochromatic images of hot, dense plasmas to be obtained in any x-ray photon energy range, over a large domain, on a single-shot basis. The device (named energy-encoded pinhole camera) is based upon the use of an array of many pinholes coupled to a large area CCD camera operating in the single-photon mode. The available x-ray spectral domain is only limited by the quantum efficiency of scientific-grade x-ray CCD cameras, thus extending from a few keV up to a few tens of keV. Spectral 2D images of the emitting plasma can be obtained at any x-ray photon energy provided that a sufficient number of photons had been collected at the desired energy. Results from recent inertial confinement fusion related experiments will be reported in order to detail the new diagnostic.

Directional bremsstrahlung from a Ti laser-produced x-ray source at relativistic intensities in the 3-12 keV range.

Front and rear side x-ray emission from thin titanium foils irradiated by ultraintense laser pulses at intensities up to ≈5 × 10(19) W/cm2 was measured using a high-resolution imaging system. Significant differences in intensity, dimension, and spectrum between front and rear side emission intensity in the 3-12 keV photon energy range was found even for 5 µm thin Ti foils. Simulations and analysis of space-resolved spectra explain this behavior in terms of directional bremsstrahlung emission from fast electrons generated during the interaction process.

Tracking propagation of ultrashort intense laser pulses in gases via probing of ionization.

We use optical interferometry to study the propagation of femtosecond laser pulses in gases. We show the measurements of propagation in a nitrogen gas jet and we compare the results with propagation in He under the same irradiation conditions. We find that in the case of nitrogen, the detailed temporal structure of the laser pulse can be tracked and visualized by measuring the phase and the resulting electron-density map. A dramatically different behavior occurs in He gas jets, where no details of the temporal structure of the laser pulse are visible. These observations are explained in terms of the ionization dynamics of nitrogen compared to helium. These circumstances make N2 gas sensitive to variations in the electric field and, therefore, allow the laser-pulse temporal and spatial structures to be visualized in detail.

Novel x-ray multispectral imaging of ultraintense laser plasmas by a single-photon charge coupled device based pinhole camera.

Spectrally resolved two-dimensional imaging of ultrashort laser-produced plasmas is described, obtained by means of an advanced technique. The technique has been tested with microplasmas produced by ultrashort relativistic laser pulses. The technique is based on the use of a pinhole camera equipped with a charge coupled device detector operating in the single-photon regime. The spectral resolution is about 150 eV in the 4-10 keV range, and images in any selected photon energy range have a spatial resolution of 5 microm. The potential of the technique to study fast electron propagation in ultraintense laser interaction with multilayer targets is discussed and some preliminary results are shown.