Investigation of efficiency of laser radiation energy transport into a planar massive target made of Al

Summary form only given. The paper is aimed at the explanation of mechanisms responsible for laser radiation absorption by a planar massive target made of Al. Absorption mechanisms of laser radiation have been studied in experiments at Prague Asterix Laser System (PALS). The investigations were performed for the first harmonic of laser radiation, four focal spot radii (40, 80, 120, and 160 μm) and the laser pulse duration of 250 ps (FWHM) at energy of 290 J. For given target irradiation conditions, the laser intensity was varied in a range of 2.6×10¹⁵-4.1×10¹⁶ Wcm^-2. The two laser beam incidence were used: normally to the target surface and at the 45° angle. Additionally, the laser beam incident normally was polarized linearly and circularly. A three-frame interferometry and measurement of crater parameters by a wax-replica technique were used as the main diagnostic tools. Two different mechanisms of laser radiation absorption were distinguished: resonance and inverse bremsstrahlung. Besides, the ratio of total electron number just after the laser pulse end to the crater volume has proved a great role of fast electrons in the laser energy transfer into the target. Investigations have also shown that the oblique incidence of laser beam reduces the resonance absorption of laser radiation.