Generation of lithium ions for ion sources using the 670.8-nm resonant transition of lithium

This article describes research on two methods of producing a 1-mm-thick singly ionized lithium plasma with an areal density up to 1×1016 cm−2. The first method relies on forming a 1-mm-thick layer of lithium vapor by ohmically pulse heating a thin film of a lithium bearing compound. The vapor is then ionized by a 670.8-nm laser light, the first resonant transition of lithium. Small scale experiments described here demonstrated average electron densities up to 5×1016 cm−3, using laser energy densities as low as 20 mJ/cm2 . The second method is direct heating, evaporation, and ionization of thin lithium bearing films on thermally insulating substrates using 670.8-nm laser light. The minimum tuned laser energy density for producing a plasma was 0.15 J/cm2 . These results are compared with measurements at other wavelengths and pulse lengths, and with models of the heating and ionization.