Interferograms of a cathode spot plasma obtained with a picosecond laser

Cathode spot plasmas as well as "plasma balls"-moving dense plasma blocks in vacuum arcs-were investigated by laser interferometry and absorption photography with high spatial (1.2 /spl mu/m) and temporal (100 ps) resolution. The discharge was initiated between a needle-type cathode and plane anode with a cathode-anode distance of a few millimeters. The duration of the discharges was about 10 ms and the current up to 100 A. Picosecond interferometry yielded spatial-temporal electron density distributions in the arc phase of the cathode spot. An absolute electron density up to 2/spl times/10/sup 26/ m/sup -3/ and plasma temperature about 1 eV in narrow plasma jets with a diameter about 5 /spl mu/m were estimated.