Oxford Centre for High Energy Density Science (OxCHEDS)

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.

Abstract:

Fast electron energy spectra have been measured for a range of intensities between 1018 and 1021 W cm-2 and for different target materials using electron spectrometers. Several experimental campaigns were conducted on petawatt laser facilities at the Rutherford Appleton Laboratory and Osaka University, where the pulse duration was varied from 0.5 to 5 ps relevant to upcoming fast ignition integral experiments. The incident angle was also changed from normal incidence to 40° in p -polarized. The results confirm a reduction from the ponderomotive potential energy on fast electrons at the higher intensities under the wide range of different irradiation conditions. © 2009 American Institute of Physics.

Abstract:

We report on experimental fast electron transport studies performed in the relativistic laser intensity interaction regime. The investigation has been carried out in the long-pulse (0.6 ps) regime relevant for the fast ignitor scheme in the inertial confinement fusion concept. Multilayer targets containing different materials were irradiated. Here we show the results concerning SiO2 or Al layers, respectively. The Kα radiation from a Cu tracer layer on the target rear side was found to be enhanced by a factor of about 8 with the irradiation of SiO2 targets with respect to the Al targets. The possible origin of this observation is discussed. © 2009 IOP Publishing Ltd.

Abstract:

The modulational instability that occurs during the interaction of a long laser pulse and its own wakefield in an underdense plasma has been studied experimentally and theoretically. Recent experiments using laser pulses that are several times longer than the wakefield period have yielded transmission spectra that exhibit a series of secondary peaks flanking the main laser peak. These peaks are too closely spaced to be the result of Raman instabilities; their origin was found to be photon acceleration of the laser's photons in the wakefield instead. In the experiments described in this paper, a laser pulse of 50-200 fs containing 300-600 mJ was focused on the edge of a helium gas jet on a 25 νm focal spot. The observed transmission spectra show evidence of both ionization blueshift and modulation by the pulse's wakefield. The transmission spectra have also been modelled using a dedicated photon-kinetic numerical code. The modelling has revealed a direct correlation between the spectral modulations and the amplitude of the excited wakefield. By comparing the measured and simulating spectra, the origin of various spectral characteristics has been explained in terms of photon acceleration. The feasibility of using this effect as a wakefield diagnostic will be discussed. © 2009 IOP Publishing Ltd.