Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the published Hugoniot Elastic Limit (HEL) for these materials. In situ x-ray diffraction has been used to directly measure the response of the shocked lattice during shock loading. Static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. In addition, experiments were conducted using a wide-angle detector to record x rays diffracted from multiple lattice planes simultaneously. These data showed uniaxial compression of Si (100) along the shock direction and three-dimensional compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed. This is evaluated to determine whether there is a phase transition occurring on the time scale of the experiments, or the HEL is much higher than previously reported. Results of the measurements are presented. (C) 2003 American Institute of Physics.

Abstract:

An experimental study of the stimulated Brillouin scattering (SBS) instability has investigated the effects of velocity gradients and kinetic effects on the saturation of ion-acoustic waves in a plasma. For intensities less than I<1.5x10(15) W cm(-2), SBS is in a linear regime and is moderated primarily by velocity gradients, while for intensities above this threshold, nonlinear trapping is relevant. Direct evidence of detuning of SBS by a velocity gradient was achieved by directly measuring the frequency of the SBS-driven acoustic wave relative to the local resonant acoustic frequency. The frequency and amplitude of the ion-acoustic wave directly responsible for SBS has been measured as a function of space using a 3omega 200 ps Thomson-scattering probe beam. Furthermore, direct evidence of kinetic effects associated with the SBS process in the nonlinear regime has been investigated through a novel use of Thomson scattering. Specifically, a measured twofold increase in the ion temperature has been linked with ion-acoustic waves that have been driven to large amplitudes by the SBS instability. Ion-acoustic waves were excited to large amplitude with a 2omega 1.2-ns-long interaction beam with intensities up to 7x10(15) W cm(-2). The measured twofold increase in the ion temperature and its correlation with SBS reflectivity measurements provides quantitative evidence of hot ions created by ion trapping in laser plasmas. These detailed and accurate measurements in well-characterized plasma conditions allow a direct test of linear and nonlinear models of the saturation of SBS. (C) 2003 American Institute of Physics.

Abstract:

We report the first direct evidence of detuning of stimulated Brillouin scattering (SBS) by a velocity gradient, which was achieved by directly measuring the frequency shift of the SBS-driven acoustic wave relative to the local resonant acoustic frequency. We show that in the expanding part of the plasma, ion-acoustic waves are driven off resonance which leads to the saturation of the SBS instability. These measurements are well reproduced by fluid simulations that include the measured flow.

Abstract:

Proton emission from 60-fs, 200-mJ high-repetition-rate tabletop-laser pulses interacting with solid targets was studied. A maximum proton energy of 1.5 MeV was measured for Mylar targets of thickness 20-40 μm. Results showed a maximum energy of 950 keV for 12μm for aluminum targets, and 850 keV for 12.5μm for copper targets.