Gianluca Gregori

Abstract:

Isochoric heating by electrons has been measured in the two limiting cases of small area thin foils with dominant refluxing and cone-long-wire geometry with negligible refluxing in the wire. Imaging of Cu K alpha fluorescence, crystal x-ray spectroscopy of Cu K shell emission, and XUV imaging at 68eV and 256eV are discussed. Laser power on target was typically 0.5 PW in 0.7ps. Heating by focused proton beams generated at the concave inside surface of a hemi-shell and from a sub hemi-shell inside a 30 degrees cone has been studied with the same diagnostic methods plus imaging of proton induced K alpha. Conversion efficiency to protons has been measured and modeled. Conclusions from the experiments, links to theoretical understanding and relevance to fast ignition are outlined.

Abstract:

A first set of shock propagation, laser-plasma interaction, hohlraum energetics and hydrodynamic experiments have been performed using the first 4 beams of the National Ignition Facility (NIF), in support of indirect drive Inertial Confinement Fusion (ICF) and High Energy Density Physics.

Abstract:

A hot, 2 to 3 keV electron temperature surface plasma was observed in the interaction of a 0.7 ps petawatt laser beam with solid copper-foil targets at intensities > 1020 W cm2. Copper K-shell spectra were measured in the range of 8 to 9 keV using a single-photon-counting x-ray charged-coupled-device camera. In addition to Kα and KΒ inner-shell lines, the emission contained the Cu Heα and Lyα lines, allowing the temperature to be inferred. These lines have not been observed previously with ultrafast laser pulses. For intensities less than 3× 1018 W cm2, only the Kα and KΒ inner-shell emissions are detected. Measurements of the absolute Kα yield as a function of the laser intensity are in general agreement with a model that includes refluxing and confinement of the suprathermal electrons in the target volume. © 2006 American Institute of Physics.

Abstract:

We present the calculation of the threshold for observation of collective plasmon modes in a solid density plasma probed with a partially coherent X-ray source. We find that at lower electron densities (ne ≲ 2 × 1023 cm-3) de-coherence effects pose a stringent limit to the effective divergence as well as bandwidth of the probe source. These results are more restrictive than the usual condition that the probed scale-length must be larger than the screening distance in the plasma. © EDP Sciences.

Abstract:

Laser-Produced plasmas at subcritical densities have proven to be efficient sources for X-ray production. In this context, we obtain experimental results from Kr and Xe gas-filled targets that were irradiated by the OMEGA (Laboratory for Laser Energetics, University of Rochester) laser. Nearly 40% of the laser energy was converted into X-rays in the L-shell-photon-energy range (≥ 1.6 keV) by a Kr-filled target. The conversion efficiency measurements were correlated with time-resolved plasma-temperature measurements done by means of a Thomson-scattering diagnostic. The measured range of temperatures, between 2-3.5 keV, is in good agreement with LASNEX radiation-hydrodynamics simulations. X-ray-cooling rates and charge-state distributions were computed using detailed atomic data from the HULLAC suite of codes. X-ray yields predicted by the cooling-rate calculations are compared to measured spectra, and good agreement is found for predictions made with highly-detailed atomic models. We find that X-ray conversion efficiency in Kr-filled targets is a strong function of temperature, and has an optimum density near 15% of the laser's critical density. © 2005 Elsevier Ltd. All rights reserved.