The formation of ring shaped laser plasmas on a metal surface

Abstract

A method to produce ring plasmas on a metal surface is presented. The ring plasma is produced by focusing a laser pulse into an annular shape. This is achieved by using a combination of a converging lens and an axicon. The axicon is a rotationally symmetric prism. The radius of the focused ring is determined by the base angle of the axicon, and the focal length of the converging lens. The experiments are performed on a titanium surface, using a 0.18 J, 10 ns, 1064 nm pulsed, from a Nd:YAG laser. The background gas is hydrogen, at pressures in the atmospheric range. The ring structure was measured using schlieren imaging and Mach Zehnder interferometry. The expansion velocities of the laser-produced plasma in the background gas were measured in two directions; parallel and perpendicular to the metallic surface. Characteristic values are 5-9(.)10(3) m/s, for the ring radius expansion, parallel to the surface, and 1.0-1.7(.)10(4) m/s, perpendicular to the surface. Characteristic electron densities of the order of 10(18) cm(-3) were measured, with hollow radial profiles. The temperature of the plasma was estimated to be between 0.1 and 0.4 eV. This ring plasma is used as precursor plasma to achieve a hollow gas embedded z-pinch.