Prof Peter Norreys FInstP;

Abstract:

Numerical studies are conducted on the electron injection into the first acceleration bucket of a laser wakefield by a weak counter-propagating laser pulse. It is shown that there are two injection mechanisms involved during the colliding laser interaction, the collective injection and stochastic injection. They are caused by the time-averaged ponderomotive force push and stochastic acceleration in the interfering fields, respectively. The threshold amplitude of the injection laser pulse is estimated for the occurrence of electron injection, which is close to that for stochastic acceleration and depends weakly upon the plasma density. The trapping of a large number of injection electrons can result in significant decay of the laser wakefield behind the first wave bucket. © EDP Sciences and Springer 2009.

Abstract:

X-ray harmonic radiation extending to 3.3 Å, 3.8 keV from Petawatt class laser-solid interactions is presented. The harmonic spectra display a relativistic limit scaling up to ∼3000th order, above which an intensity dependent scaling roll-over is observed. Highly directional beamed emission for harmonic photon energy hv > 1 keV is found to be into a cone angle < 4°, significantly less than that of the incident laser cone (20°). © EDP Sciences and Springer 2009.

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.