Barium loss from the electrode of a low pressure Hg-Ar discharge at ignition

Summary form only given, as follows. Using laser absorption, we have continued our measurements of the spatial and time dependent barium ion density in the vicinity of a thermionic cathode for a low pressure mercury-argon discharge. Barium is constituent of the emissive coil coating in lamps and other discharge devices. Measurements were performed during ignition of the discharge, and can be used to determine the flux of barium from the coil during starting. Results were obtained for both d.c. and high frequency (/spl sim/40 kHz) excitation. The experimental method utilizes multiple parallel laser beams traversing chords of the cylindrical 25 mm diameter glass discharge chamber in the electrode region. The dye laser is tuned to a resonance line of the singly ionized barium ion. Hence, the attenuation of the beams is a measure of the chord averaged barium ion density. The data is Abel inverted to obtain radially dependent density information. Data are acquired from the moment of ignition until stable thermionic operation of the electrode is attained.