CO2 Laser Induced Breakdown and Heating of Hydrogen in a Magnetic Mirror Field Geometry

Abstract

A 1 GW, 125 J CO2 laser pulse focussed by a 10 cm f.l. parabolic mirror has been used to induce breakdown and heating in 15 Torr hydrogen along the axis of a short 50 kG magnetic mirror field. Evolution of the breakdown plasma thus produced has been studied by high speed photography, ruby laser Thomson scattering, and a Mach–Zehnder interferometer. Results indicate an efficient radial confinement of plasma by the mirror field with an electron temperature much enhanced over that resulting from a fieldfree breakdown. Interferometric measurements show that breakdown results in a cylindrical plasma with an electron density minimum on axis in both cases. A monitor of the transmitted CO2 laser pulse indicates very incomplete coupling of laser radiation into the plasma column with the field applied, presumably due to refraction losses at the entrance boundary of the breakdown.