A New Model of Crater Formation by Arc Spots

Abstract

SummaryThe forces acting on the lcathode arc spot surface and removing the molten layer from the crater bottom are composed mainly of the ion pressure, the neutral gas pressure and the evaporation recoil whilst electrostatic forces diminish the effective pressure that is in the order of some loo dyn/cm?. The motion of the liquid layer caused by these forces is treated with the hydrodynamic equations. A simple solution exists in the special casc of constant layer depth, that is achieved it few nanoseconds after spot formation. From this model the layer depths (some 0.1 pm) and the ejection velocities a t the crater rims (few 10* cm/a) tire calculated. The real spot velocity agrees with the velocity of the melting front below the spot surface, but because of the stochastic character of the spot motion the apparent velocity decreascs with growing observation time intervals At according to At-'P. Basic ConceptionsThe tracks left behind by moving vacuum arc spots on the cathode surface consist of craterlike structures, obviously resulting from melting processes and forced motion of liquid metal. These craters are easily detectable on scanning microscope pictures (e.g. [l, 2, 51). The development of such structures may become intellegible by an arc spot model recently proposcd [ 3 , 41. We have to expect the following succession of events :1. Spot formation by breakdown processes, initiated in most cases by explosive emission of a protrusion, 2. Heating of the spot surface (mainly by ionimpact andneutralization) up to themelting temperature, 3. Progress of the melting front into the cathode metal below the spot surface, formation of a liquid pool, 4. Acceleration of the liquid layer towards the borders of the spot, mainly due to ion pressure forces, leading to an excavation of the spot and to crater formation, to an overflow of the metal a t the rims of the crater, to the ejection of splashes and to the rise of emission tips, 5. Explosion of sharp emission sites (because of localized field enhancement and rapid Joule heating) on the crater rims result in a shift of the spot center and in the spot motion. Plasmaphrsik

Formation time and heating mechanism of arc cathode craters in vacuum

“…An uninterrupted movement without the magnetic field occurs generally at random. According to Hantzsche (1977) the average displacement x during the time t is then given by x2…”

Hydrodynamics of the Molten Metal During the Crater Formation on the Cathode Surface in a Vacuum Arc

“…The time of formation of such an elemental crater, was investigated for near-threshold arc currents [6]. A microcrater is formed as a result of displacement of the molten cathode metal due to the pressure of the cathode spot plasma [7]- [10]. The extrusion of the molten metal from the crater results in the formation of microjets, which afterward break into microdrops.…”

Cathode Heating by Vacuum Arcs: A Survey

“…Indeed, craters can be formed already within 3 nanoseconds [27, 28, 321, whereas ECKER [2], RICH [15] and KULYAPIN [33] calculate times of order microseconds. MCCLURE [ll], HANTZSCHE [8] and DAALDER [12] obtain values around 100 ns (or somewhat below). Also these times are still large.…”