MEASUREMENT OF CATHODE SPOT PARAMETERS WITH PULSED-LASER DIAGNOSTICS

The cathode spot formation in air within the first 170 ns was investig ated by laser absorption photography and ps-pulse interferometry. The discharge was initiated between electrodes made from Ag or Pd with cat hode-anode distance below 300 mu m, the are duration was some millisec onds, and the are current 5-10 A. Picosecond holographic interferometr y and momentary absorption photography yielded spatial-temporal densit y distributions in the ignition phase of the cathode spot. An absolute electron density value on the order of 4 x 10(26) m(-3) has been foun d. In contrast to vacuum, the cathode spot plasmas broaden little with increasing distance from the cathode, thus narrow plasma channels are observed in the vicinity of the cathode surface having diameters <20 mu m.