Interaction of high intensity picosecond laser pulses with large preformed plasmas

The interaction of short (1-2 ps) laser pulses with solid targets at irradiances of over 10¹⁶ Wem ^-2, in the presence of a substantial prepulse has been investigated High absorption of laser energy is found even at high angles of incidence, with evidence for a resonance absorption peak being found for S. P. and circular polarizations. It is considered that this may be a result of refraction and beam filamentation. which causes loss of distinct polarization Measurements of hard X-ray emission (̃ 100 keV) confirm a resonance absorption type peak at 45-50°. again for all three cases. Typically, 5-15% of the incident light is back-reflected by stimulated Brilloum scatter, with spatially resolved spectra showing evidence of beam hot-spois at high intensity. The possibility that filamentation and refraction of the beam can explain the lack of polarization dependence in the absorption and hard X-ray emission data is discussed.