Jobst Verleger

Detection of metal vapour in the high-current phase of a pseudospark switch by resonance absorption of laser light

The electron emission mechanism which is active at the cathode of a pseudospark switch leads to thermal overloading of emission sites followed by the evaporation of cathode material. As a consequence, metal vapour should be present very shortly after the beginning of current flow. However, it has never been possible to observe metal vapour by laser-induced fluorescence (LIF) prior to the cessation of current. In the present work resonance absorption of laser light was used to detect molybdenum vapour from the cathode during the high-current phase of a pseudospark switch. To observe the narrow-band absorption the laser light which had passed the switch was introduced into a molybdenum hollow cathode lamp where it caused fluorescence from molybdenum vapour. The intensity of the fluorescence light from the hollow cathode lamp was proportional to the intensity of the laser light left after the absorption process in the pseudospark. For a current amplitude of 12 kA and a pulse duration of , it is shown that molybdenum vapour is definitely present shortly after the current maximum, well before the current stops flowing. The neutral vapour density in the high-current phase is estimated to amount to at least . It is concluded that LIF fails to detect metal vapour prior to the cessation of current, mainly because the fluorescence process is severely disturbed by electronic collisions which deplete the upper fluorescence level.