DETECTION OF METAL VAPOR 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 ps eudospark switch leads to thermal overloading of emission sites follow ed by the evaporation of cathode material. As a consequence, metal vap our 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 mo lybdenum vapour from the cathode during the high-current phase of a ps eudospark switch. To observe the narrow-band absorption the laser ligh t 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 p roportional to the intensity of the laser light left after the absorpt ion process in the pseudospark. For a current amplitude of 12 kA and a pulse duration of 1.6 mu s, it is shown that molybdenum vapour is def initely present shortly after the current maximum, well before the cur rent stops flowing. The neutral vapour density in the high-current pha se is estimated to amount to at least 6-8 x 10(17) m(-3). It is conclu ded that LIF fails to detect metal vapour prior to the cessation of cu rrent, mainly because the fluorescence process is severely disturbed b y electronic collisions which deplete the upper fluorescence level.