Time-resolved visible spectroscopy of laser-produced lithium plasmas

We have measured time-resolved visible spectra emitted from a plasma formed when the output from a dye laser irradiates solid lithium. Such a plasma has potential as a source of lithium ions for ion-driven inertial confiement fusion, and it also provides a useful source for developing diagnostics. The laser delivered 0.5 Joules onto a 2-7 mm diameter spot, with a 900-ns pulse length. Experiments were performed with the wavelength tuned to the Li I 2s-2p resonance line at 6708 /angstrom/ and off-resonance at 6728 /angstrom/. The target was a 500 to 1000 /angstrom/ thick Li film, vacuum evaporated in situ onto a substrate. The light from the plasma was coupled into the entrance slit of a 1-m Czerny-Turner spctrometer, and the output from the spectrometer was focused onto the input slit of a steak camera. The electron density was obtained from Stark broadened widths of Li I 2p-4d, 2p-5d, 2p-4s, and H I 2p-4d. An irradiance of 2 x 106 wcm2 at 6708/angstrom/ resulted in a peak electron density of 3.9 x 1017 cm/sup /minus/3/. The density decreased at lower irradiance, with an intensity threshold of 5 x 105 Wcm2 for producing an ionized plasma. The threshold for producing a plasma was higher with the laser tuned off resonance, although high density lithium plasmas could still be formed at relatively lower laser irradiance. 6 refs., 4 figs