Laser-initiated discharge channels in sodium vapor

As the preliminary experiments for the propagation of ion beams, we produced electrical discharge channels in alkali metal vapors by resonant laser lights. A z-discharge channel was initiated by a laser beam which was tuned to the sodium D resonance line (λ=589 nm). The heated discharge chamber (50 mm in inner diameter) was filled with the mixture gas of sodium vapor and N2. The total pressure of the gas was 0.15 Torr, and the maximum temperature was 320°C. The temperature corresponded to the sodium vapor pressure of 0.03 Torr. Applying a voltage sufficiently lower than the self-breakdown voltage (0.7 kV), the discharge channel of ∼40 cm in length was initiated by the laser beam with the spectral power density of 14 kWcm⁻²nm⁻¹. A DC breakdown voltage (minimum voltage allowing laser - initiation) was decreased with the increase of the spectral power density. An initial-stage behavior of the discharge was observed with a streak-camera. It was clarified that the laser-initiated discharge channel was produced just along the laser beam path. Similar experiments with a laser beam tuned to the potassium resonance line (λ=769.8 nm), and a UV-laser beam (λ=308 nm) to ionize cesium atoms are described in short.