Techniques And Initial Results From An X-Ray Laser Experiment Using Laser-Vaporized Copper Foil Plasmas

Abstract

A series of exploratory experiments recently carried out at NRL were directed towards the possible utilization of laser-heated ultra-thin copper films for generating elongated plasmas of sufficient uniformity to produce amplification on 3s-3p transitions in neon-like copper ions. The NRL Pharos III laser operating at 140-335 J in 2-6 ns pulses was used as a driver. Variations were made in the plasma length, the laser energy and pulse shape, and the copper thickness in order to optimize the gain-medium conditions. A primary necessity was to assure on each attempt an accurately-aligned vuv grazing-incidence spectrograph. Using space-resolved x-ray crystal spectroscopy and pinhole photography as auxiliary diagnostics, axial homogeneity as well as front/rear symmetry were measured. The electron temperature in the Cu XX plasma, as estimated from intensity ratios of 2p-nd transition x-ray lines, was found to increase with copper thickness. Various explanations for the lack of measureable gain in these inital tests are discussed. A novel slotted copper foil (thicker) target design was also tested and showed similar characteristics to the thin copper film targets. Also, spectral features from a selenium target exposure are described.