Gianluca Gregori

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:

The construction of short-pulse tunable soft x-ray free electron laser sources based on the self-amplified spontaneous emission process will provide a major advance in capability for dense plasma-related and warm dense matter (WDM) research. The sources will provide 1013 photons in a 200-fs duration pulse that is tunable from approximately 6 to 100 nm. Here we discuss only two of the many applications made possible for WDM that has been severely hampered by the fact that laser-based methods have been unavailable because visible light will not propagate at electron densities of ne ≥ 1022cm-3. The next-generation light sources will remove these restrictions. © 2003 Optical Society of America.