ABSORPTION AND HOT-ELECTRON PRODUCTION BY HIGH-INTENSITY FEMTOSECOND UV-LASER PULSES IN SOLID TARGETS

The interaction of femtosecond KrF-laser pulses with plasmas of vario us solid target materials has been studied up to intensities exceeding 10(18) W/cm(2). Absorption measurements were performed for p- and s-p olarized laser light and as a function of the laser intensity and the angle of incidence. They reveal substantial absorption up to 70% even at intensities in excess of 10(18) W/cm(2). The results have also been compared to simulations of the absorption at high intensities and, in particular, the peaking of the absorption for large angles of inciden ce (70 degrees-80 degrees) appears to be consistent with the anomalous skin effect as an important contribution to the total laser pulse abs orption. X-ray spectra were measured in the keV range (i.e., between 6 .5 and 8.4 Angstrom) and in the soft-x-ray region (i.e., between 25 an d 400 Angstrom). The electron density and temperature of the plasma ha s been estimated by comparison of the experimental spectra with spectr al simulations. A systematic study of the hot electrons produced by 24 8-nm light is presented. Targets consisting of an Al layer on a Si sub strate have been used to determine the hot electron yield and the corr esponding energy. The K-alpha line emission produced by the hot electr ons has been observed as a function of the Al-layer thickness. The mea surements have been compared to simulations. The estimated hot electro n temperature similar to 8 keV is considerably lower than that deduced from experiments using lasers of longer wavelength and comparable int ensities. Scaling indicates that 0.25-mu m lasers can simultaneously f ulfill the requirements for both intensity and hot electron temperatur e for the ''fast ignitor.''