Time-resolved visible spectroscopy of laser-produced lithium plasmas

Abstract

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 confinement fusion, and it also provides a useful source for developing diagnostics. The laser delivered 0.5 J onto a 2–7‐mm‐diam spot, with a 900‐ns pulse length. Experiments were performed with the wavelength tuned to the Li i 2s–2p resonance line at 6708 Å and off resonance at 6728 Å. The target was a 500–1000‐Å‐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 spectrograph, and the output from the spectrograph was focused onto the input slit of a streak camera. The electron density was obtained from Stark‐broadened widths of Li i 2p–4d, 2p–5d, and 2p–4s, and H i 2p–4d. An irradiance of 2×10⁶ W/cm² at 6708 Å resulted in a peak electron density of 3.9×10¹⁷ cm⁻³. The density decreased at lower irradiance, with an intensity threshold of 5×10⁵ W/cm² 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 low laser irradiance.