Laser energy transformation to shock waves in multi-layer flyers

Abstract

Investigations of powerful laser pulse action on planar flyer targets consisting of the layers of different materials are of importance from the basic as well as the applied physics point of view. One important aspect of this research deals with optimization of inertial fusion targets design. Here, the role of a thin heavy metal layer as a protector against preliminary heating of compressed thermonuclear fuel by thermal X-ray radiation can be mentioned as one particular topic to be properly understood. In this paper, the results of our studies of such a thin layer influence on the laser-produced energy deposition in the flyer foils as well as on the hydrodynamic motion of the foils as a hole will be reported. A 0.4 μ m thick gold layer was located between an aluminum layer of 6 μ m thickness and mylar layer with thickness of 2.5 μ m, used here as an ablator. For comparison, the flyer target without the gold layer but of the same area density was employed. Two different target constructions were used: (1) with a 2 mm gap separating the foil and the massive targets for measuring the foil velocity and (2) with gaps of 50, 100 and 200 μ m for laser energy transfer efficiency measurements. Targets were irradiated by laser beam energies of ∼100 J in the case of the first harmonic, and by laser beam energies ∼120 J in the case of the third one using the Prague Asterix Laser System iodine laser. The interaction spot radius of 200 μ m and the laser pulse duration of ∼250 ps were employed in these experiments. A three-frame interferometric and shadowgraphic system was set-up as to measure velocities of the rear side of the foils and to determine electron density distributions at different stages of plasma evolution. Volumes of craters produced by collisions of accelerated foils with a massive aluminum block were used as an indicator of the laser energy transfer efficiency into the foils of both types. These experiments have shown that the presence of the thin gold layer causes a decrease of the flyer velocity by a factor of ∼1.2 for both the harmonics. However, in the case of the first harmonic, the energy transfer into the flyer with the gold layer is higher (compared with the flyers without the gold layer) only for the targets with smaller gaps. In the case of the third harmonic, about 10% decrease in the amount of energy transferred into the foil with the gold layer was observed.