Burkhard Jüttner

Cathode spots of electric arcs

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??m, 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??m, level B associated with spot fragments having a size of 10-20??m 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??s. 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.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.

Pulsed dye laser diagnostics of vacuum arc cathode spots

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 mu m. A 0.4 ns laser pulse (tunable, gamma =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 mu m. They appear and disappear on a nanosecond time scale. The plasma density of the microspots was estimated to be greater than (3-6)*10/sup 26/ m/sup -3/. >

Characterization of the Cathode Spot

A general description of arc cathode spots is given with respect to structure, size, and temporal behavior. The density of the metal vapor plasma outside the spot is presented as a function of the coordinates, calculated on the basis of ion currents flowing to screens and probes. Extrapolation to the region within the spot indicates plasma densities in excess of 1025 m-3. Recent studies of spot current density suggest values of about 1012 A/m2. In this connection, the problem of the electrical conductivity of the spot plasma is discussed. Finally, experiments on spot lifetime and models of crater formation are reviewed.

The retrograde motion of arc cathode spots in vacuum

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??m, respectively. The magnetic flux density amounted to B = 0.4?T and the arc currents to 2-100?A. For times <1??s random displacement occurs on a time scale <100?ns. At intervals of about 4??s, jumps of the spot are observed over distances of 50-300??m 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 = ? 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??m, respectively. No reversal of the motion has been observed at elevated temperatures up to 2100?K.

Formation time and heating mechanism of arc cathode craters in vacuum

With clean cathodes in UHV it is shown experimentally that (i) the arc craters are formed successively and displaced without spatial interruption, (ii) there are only a few active craters at one instant for currents below the spot-splitting limit (generally 1-2), and (iii) the crater formation is much faster than is compatible with the heat conduction time scale, the formation time amounting to only a few nanoseconds. From these observations and with the measured values of crater radii and formation times the heating mechanism of the craters is estimated. The calculations show that Joule heating is insufficient to explain the short time scale, therefore ion impact heating is concluded to be the dominant process. Also this energy source is effective only by heating thin layers of a well defined thickness (about 0.1 mu m) which are removed immediately after melting. Therefore, the melting front proceeds faster into the interior than by heat conduction. This model explains the high heat conduction losses at the cathode as measured by Daalder (1977), and also the reason for the arc spot movement.

Structure and dynamics of high-current arc cathode spots in vacuum

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.

Time-Resolved Measurements of the Parameters of Arc Cathode Plasmas in Vacuum

Plasma parameters of carbon arcs in vacuum have been measured with high temporal and spatial resolution (1 ¿s and 10 ¿m, respectively). Near the surface the electron temperature was ¿1 eV. It increased to values of about 10 eV for distances > 10 mm by anomalous heating due to ion sound turbulence. The average electron density varies with the distance r as r-2. Values of about 1025 m-3 have been found at r = 15 ¿m.

Time dependence of vacuum arc parameters

Time-resolved investigations of the expanded plasma of vacuum arc cathode spots are described, including the study of the ion charge state distribution, the random cathode spot motion, and the crater formation. It was found that the ion charge state distribution changes over a timescale on the order of hundreds of microseconds. For the random spot motion two timescales were observed: a very short spot residence time of tens of nanoseconds which gives, combined with the step width, the diffusion parameter of the random motion, and a longer timescale on the order of 100 mu s during which the diffusion parameter changes. Crater formation studies by scanning electron microscopy indicate the occurrence of larger craters at the end of crater chains. The existence of a timescale much longer than the elementary times for crater formation and spot residence can be explained by local heat accumulation. >

The dynamics of arc cathode spots in vacuum: new measurements

Arc cathode spots in vacuum with Ti- and Cu-cathodes have been studied by using the high-speed framing camera IMACON 468 in the time range down to 10 ns. The spots exhibited an inner structure that indicates the existence of fragments. At currents of 70 A their number was 5 - 7. These structures showed high dynamics, attracting and repelling one another with speeds up to . This was associated with quasi-periodic fluctuations of the spot brightness, yielding a new explanation for the brightness fluctuations reported in a preceding paper. One of the fluctuations, occurring every , had a most probable duration (width of the light pulse) of 200 - 300 ns. The spatial resolution, per pixel, was sufficient to allow observation of surface craters in situ. So it turned out that the fragments are located at crater rims and that the crater structures may change in times <100 ns. As a consequence of periodic attracting and repelling of the fragments, the spot displaces at random. The mean square displacement was measured to be for observation times of 100, 200 and 500 ns. From the time scale of surface changes it is concluded that the main energy source for crater formation is surface bombardment by the plasma ions rather than Joule heating of the crater volume.

Influence of residual gases on cathode spot behavior

A residual gas in a vacuum arc chamber influences the behavior of the arc by two effects: it changes the state of the cathode surface, in particular the surface cleanness, and it influences the interelectrode plasma. Experiments are summarized dealing with the influence of the residual gas on the arc parameters in a pressure range of 10/sup -6/-10/sup -5/ Pa. With increasing pressure, general tendencies are a decrease in the fluctuations of the burning voltage, in the chopping current, and in the current density, and an increase in the arc lifetime, spot velocity, and spot diameter. The conditions at the cathode surface are decisive for the spot behavior and not the pressure. Surface contaminations render the arc more stable. The transition between the so-called cathode spot type 1 (on contaminated surfaces) and type 2 (on clean surfaces) was found to be smooth rather than abrupt. >