Gianluca Gregori

Abstract:

We have previously reported the experimental discovery of a second shock forming ahead of a radiative shock propagating in Xe. The initial shock is spherical, radiative, with a high Mach number, and it sends a supersonic radiative heat wave far ahead of itself. The heat wave rapidly slows to a transonic regime and when its Mach number drops to two with respect to the downstream plasma, the heat wave drives a second shock ahead of itself to satisfy mass and momentum conservation in the heat wave reference frame. We now show experimental data from a range of mixtures of Xe and N-2, gradually changing the properties of the initial shock and the environment into which the shock moves and radiates (the radiative conductivity and the heat capacity). We have successfully observed second shock formation over the entire range from 100% Xe mass fraction to 100% N-2. The formation radius of the second shock as a function of Xe mass fraction is consistent with an analytical estimate.

Abstract:

The expansion of shock waves has been studied in mediums with different opacities and heat capacities, varied in systematic ways by mixing xenon with nitrogen keeping the mass density constant. An initial shock is generated through the brief (5 ns) deposition of laser energy (5 J) on the tip of a pin surrounded by the xenon-nitrogen mixture. The initial shock is spherical, radiative, with a high Mach number, and it sends a supersonic radiatively driven heat wave far ahead of itself. The heat wave rapidly slows to a transonic regime and when its Mach number drops to ∼2 with respect to the downstream plasma, the heat wave becomes of the ablative type, driving a second shock ahead of itself to satisfy mass and momentum conservation in the heat wave reference frame. The details of this sequence of events depend, among other things, on the opacity and heat capacity of the surrounding medium. Second shock formation is observed over the entire range from 100% Xe mass fraction to 100% N2. The formation radius of the second shock as a function of Xe mass fraction is consistent with an analytical estimate. © 2006 American Institute of Physics.

Abstract:

We describe measurements characterizing the interaction of ultra-high intensity Petawatt laser pulses with solid targets. Experiments were performed on the Petawatt laser at RAL, and the Titan laser at LLNL. © 2006 Optical Society of America.

Abstract:

We report recent progress in x-ray diagnosis of laser-plasma experiments using imaging plates. Imaging plates are photostimulable phosphor screens [BaF(Br0.85, I0.15): Eu2+] deposited on flexible metal or plastic substrates. We applied imaging plates to x-ray microscopy of inertial confinement fusion experiments. Self-emission x-ray images of imploded cores were obtained successfully with high-magnification, target-mounted pinholes using imaging plates as detectors. Imaging plates were also used in ultraintense laser experiments at the Rutherford Appleton Laboratory, where small samarium foils were irradiated by high intensity laser pulses from the Vulcan laser system. K-shell x rays from the foil (similar to 40 keV) were used as a line x-ray source for one-dimensional microscopic radiography, and the performance of imaging plates on high-energy x-ray backlit radiography experiments was demonstrated by imaging sinusoidal grooves of 6 mu m amplitude on a Au foil. Detailed K-shell spectra from Cu targets were also obtained by coupling an imaging plate with a highly ordered pyrolytic graphite crystal spectrometer. The performance of the imaging plates as evaluated in actual laser plasma experiments is presented. (c) 2006 American Institute of Physics.

Abstract:

This article discusses the design of a novel time resolved von Hamos Bragg spectrometer to provide spectra in the region around the titanium K-alpha and He-alpha lines. The instrument consists of a highly oriented pyrolitic graphite mosaic crystal coupled to a picosecond x-ray streak camera. Measurements of the time dependent behavior from Ti foils illuminated with intense laser pulses can be used to improve the understanding of recombination dynamics, electron transport, and phase transitions in strongly coupled dense plasma. This is important for the modeling of the compression phase in inertial confinement fusion research and the study of astrophysical environments. (c) 2006 American Institute of Physics.