Gianluca Gregori

Abstract:

A dual-channel, curved-crystal spectrograph was designed to measure time-integrated x-ray spectra in the ∼1.5 to 2 keV range (6.2-8.2 Å wavelength) from small-mass, thin-foil targets irradiated by the VULCAN petawatt laser focused up to 4× 10 20 W/ cm 2. The spectrograph consists of two cylindrically curved potassium-acid-phthalate crystals bent in the meridional plane to increase the spectral range by a factor of ∼10 compared to a flat crystal. The device acquires single-shot x-ray spectra with good signal-to-background ratios in the hard x-ray background environment of petawatt laser-plasma interactions. The peak spectral energies of the aluminum He α and Ly α resonance lines were ∼1.8 and ∼1.0 mJ/eV sr (∼0.4 and 0.25 J/Å sr), respectively, for 220 J, 10 ps laser irradiation. © 2009 American Institute of Physics.

Abstract:

We present calculations of the free-free opacity of warm, solid-density aluminum at photon energies between the plasma frequency at 15 eV and the L-edge at 73 eV, using both density functional theory combined with molecular dynamics and a semi-analytical model in the RPA framework which includes exciton contributions. As both the ion and electron temperature is increased from room temperature to 10 eV, we see a marked increase in the opacity. The effect is less pronounced if only the electron temperature is allowed to increase, while the lattice remains at room temperature. The physical significance of these increases is discussed in terms of intense light-matter interactions on both femtosecond and picosecond time scales. © 2009 Elsevier B.V. All rights reserved.

Abstract:

We have performed measurements of the radiation and the hot electron temperature in sub-millimetre size hohlraums driven by a high intensity short-pulse laser. The results indicate that radiation temperatures ∼80 eV can be obtained with ∼20 J of laser energy delivered on target. Radiation-hydrodynamics simulations indicate an absorption into thermal X-rays of ≲1-2%, with peak temperatures similar to those measured experimentally. Crown Copyright © 2009.

Abstract:

We present results for the ionic structure in dense, moderately to strongly coupled plasmas using two models: the mean spherical approximation (MSA) and the hypernetted chain (HNC) approach. While the first method allows for an analytical solution, the latter has to be solved iteratively. Independent of the coupling strength, the results show only small differences when the ions are considered to form an unscreened one-component plasma (OCP) system. If the electrons are treated as a polarizable background, the different ways to incorporate the screening yield, however, large discrepancies between the models, particularly for more strongly coupled plasmas. © 2009 IOP Publishing Ltd.

Abstract:

Experiments show that application of laser smoothing schemes including smoothing by spectral dispersion and polarization smoothing effectively suppresses stimulated Raman scattering from a 2ω (527 nm) laser beam in a low average-gain plasma with a steep density gradient. Full-wave simulations reproduce the observed trends in the data and show that the scattering reduction is an indirect result of suppressing active filamentation. © 2009 American Institute of Physics.