A review is given on arc cathode spots, mainly based on the investigation of arcs in a vacuum with cold cathodes. For the latter and after a short description of general features and theoretical concepts, experiments are presented that study the temporal and spatial behaviour of the spots with high time and space resolution of less than 10 ns and less than 5 µm, respectively. With the help of these observations the various spot types described in the literature are ordered into three levels: level A corresponding to the proper spot with typical diameters of 50-100 µm, level B associated with spot fragments having a size of 10-20 µm and level C comprising a substructure of the fragments. The structures undergo periodic fluctuations of brightness and position with characteristic times that can be arranged in a hierarchy from a few nanoseconds through about 100 µs. The analysis of these fluctuations shows that the spot operates in cycles that include both extremely non-stationary periods with time constants of less than 10 ns and more stationary periods in the microsecond range. In the presence of an external magnetic field, the latter periods lead to unstable plasma configurations that give rise to retrograde motion. Finally, for vacuum arc spots the basic parameters are summarized. After that, the peculiarities of spots in gases with cold electrodes are discussed, followed by a presentation of spots with hot cathodes at high pressures.

Ball-like plasmoids were generated from discharging a capacitor bank via a water surface. In the autonomous stage after current zero they have diameters up to 0.2 m and lifetimes of some hundreds of milliseconds, thus resembling ball lightning in some way. They were studied by applying high speed cameras, electric probes, calorimetric measurements, and spectroscopy. The plasmoids are found to consist of a true plasma surrounded by a cold envelope. Decreasing electron densities in the order of 10 20 m −3 to 10 22 m −3 were measured from Stark broadening in the initial (formation) phase. The electron temperature is estimated to be 2000-5000 K during most of the plasmoid's lifetime. The temperature of the neutral particles can exceed 1300 K. Calcium hydroxide molecular band emission is the major source of visible radiation in the autonomous phase. Chemiluminescence reactions between dissociation products of water and dissolved calcium are proposed as a source for this emission. The plasmoid's colder boundary layer consists of electric double layers that may attribute to the characteristic shape of the balls.

The ignition and arc phases of vacuum arcs were investigated using differential dye laser absorption photography with simultaneous high spatial (micrometer) and temporal (nanosecond) resolution. The discharge duration was 800 ns, the current 50-150 A, the electrode material copper, and the cathode-anode distance less than 50 μm. A 0.4 ns laser pulse (tunable, γ=480-530 nm) was used to obtain momentary absorption photographs of the cathode region. During ignition, an optically thick anode plasma expanded toward the cathode, decaying within 25 ns after bridging the electrode gap. In the arc phase, a fragmentary structure of the cathode spots was observed in situ for the first time. The microspots have a characteristic size of 5-10 μm. They appear and disappear on a nanosecond time scale. The plasma density of the microspots was estimated to be greater than (3-6)×1026 m-

Experiments are reported on the number and displacement velocity of arc spots on CuCr and Cu cathodes in the current range 40 - 1500 A. The spot number was found to increase linearly with current. The average current per resolvable spot amounted to for CuCr and for Cu. For times after ignition random spot displacement R was observed, having mean square values of for Cu and for CuCr. The Cu spots showed brightness fluctuations with intervals of . Because of this obvious dynamics, the theoretical models of the spot plasma must be time-dependent. A self-consistent theoretical description of Cu and Cr plasmas is given, yielding the cathode temperature, plasma density, electric field strength, current density and plasma velocity in the time range 10 ns to 3 ms.

Experiments are reported on the retrograde arc spot motion on copper and tantalum cathodes in vacuum in the presence of a magnetic field. The spots are imaged with time and space resolutions of <100 ns and <10 µm, respectively. The magnetic flux density amounted to B = 0.4 T and the arc currents to 2-100 A. For times <1 µs random displacement occurs on a time scale <100 ns. At intervals of about 4 µs, jumps of the spot are observed over distances of 50-300 µm in the retrograde direction, thus yielding macroscopic velocities of about 50 m s-1. The jumps are preceded by the ejection of plasma jets in the retrograde direction, having average velocities of about v = 5 km s-1. New spots are formed exactly in the jet direction. The jets are explained by instabilities in the magnetically confined spot plasma, and the spot formation by electric fields E⃗ = v⃗×B⃗ within the jets. The jets are ejected in periods of enhanced plasma production caused by the inner spot processes, i.e., by the dynamics of fragments and cells, having diameters of ⩽20 and ⩽10 µm, respectively. No reversal of the motion has been observed at elevated temperatures up to 2100 K.