Intensity-dependent characteristics of a picosecond laser-produced Cu plasma

The interaction of a I ps laser pulse of moderate contrast ratio (similar t o 10(4)) with a Cu target is investigated at laser intensities I of up to 3 x 10(16) W cm(-2) with the use of corpuscular, x-ray and optical diagnosti cs. Intensity-dependent characteristics of ion and x-ray emission as well a s reflectivity, electron temperature and average charge state of plasma pro duced by the laser pulse are presented and discussed. It is found that (a) the specular reflectivity dependence on intensity for a moderate-contrast u ltrashort (less than or equal to 1 ps) pulse is qualitatively and quantitat ively different from that for a high-contrast ultrashort pulse; (b) the sca ling laws for parameters of thermal ions are essentially different from tho se for fast ions; (c) the soft x-ray yield scales as I-alpha with alpha app roximate to 2-2.5 in the intensity range 10(14)-3 x 10(16) W cm(-2); and (d ) electron temperature scales as I-0.44 at 10(15) < I < 3 x 10(16) W cm(-2) .