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The mechanism of the emission of neutral atoms of the cathode material into the discharge gap of a microsecond low-current vacuum arc with a liquid Ga cathode has been investigated by the method of subnanosecond resonance laser interfero- and shadowgraphy. It has been shown that the cathode material vaporization has a pronounced nonstationary character and occurs both isotropically and in the form of constricted weakly ionized jets with the atom concentration in a jet over [10.sup.17] [cm.sup.-3]. Index Terms--Cathode spot, resonance laser interferography, vacuum arc.

The temporal evolution of X-ray emission of laser-induced discharges was investigated by means of a picosecond X-ray streak camera. Point-like structures with small divergence and thin hot layers of intense X-ray radiation with life time from 30 ps to 1.5 ns in a spectral range 100 eV-10 keV have been found in a comparatively low voltage vacuum discharge (U [similar to] 150 V-2.7 kV) initiated by picosecond laser beam. The radiation of laser-induced breakdown was investigated just after breakdown as well as with a delay time (up to 20 ns) relative to the ignition point by the laser beam in order to eliminate the X-ray radiation coming from the laser-produced plasma.

Cathode spot formation in laser-induced breakdown in vacuum was investigated by laser absorption photography with high spatial (0.5 [[micro]meter]) and temporal (100 ps) resolution. The discharge was initiated between Cu electrodes with a cathode-anode distance of 15-250 [[micro]meter]. The duration of pulsed discharges was 750 ns and dc discharges some milliseconds; the current was below 10 A. Picosecond momentary absorption photography yielded spatial-temporal density distributions in the ignition phase of the cathode spot. An absolute electron density >5 x [10.sup.26] [m.sup.-3] in narrow plasma fragments with a diameter smaller than 5 [[micro]meter] was estimated. Mathematical modeling has satisfactorily explained the formation of the narrow plasma channel due to the bulk current self-focusing, as well as due to the generation of nonstationary emissive centers at the moving boundary of the expanding cathode spot plasma. Index Terms - Absorption photography, cathode spot, laser-induced discharge, laser-induced plasma, vacuum arc.

The cathode spot formation in air within the first 170 ns was investigated by laser absorption photography and ps-pulse interferometry. The discharge was initiated between electrodes made from Ag or Pd with cathode-anode distance below 300 [[micro]meter], the arc duration was some milliseconds, and the arc current 5-10 A. Picosecond holographic interferometry and momentary absorption photography yielded spatial - temporal density distributions in the ignition phase of the cathode spot. An absolute electron density value on the order of 4 x [10.sup.26] [m.sup.-3] has been found. 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

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/[micro]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 x [10.sup.26] [m.sup.-3] and plasma temperature about 1 eV in narrow plasma jets with a diameter about 5 [micro]m were estimated. Index Terms--Cathode spot plasma, interferometry, pulsed laser diagnostics, shadowgraphy, vacuum arcs.

The temporal evolution of X-ray emission of laser-induced discharges was investigated by means of a picosecond X-ray streak camera. Point-like structures with a divergence of order [10.sup.-7] and thin hot layers of intense X-ray radiation with life time from 30 ps to 1.5 ns in spectral range 100 eV-10 kev have been found in a comparatively low voltage vacuum discharge (U = 150 V-2.7 kV) initiated by picosecond laser beam. Computer simulation shows that such X-ray generation is connected with a matter transition into extreme states when in local microvolumes of dense current carrying plasma the pressures can exceed values of 5 Mbar and the specific internal energies [Epsilon] [greater than or equal to] 1 MJ/g. Index Terms - Cathode spot, extreme states of matter, laser-induced discharge.