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Investigations of the diffuse plasma expanding from cathode arc spots into a vacuum have revealed some unexpected properties calling for physical explanation. A theoretical model of such plasmas which is based on one-dimensional time-independent hydrodynamic two-fluid equations can be solved analytically in the form of asymptotic power series approximately describing the plasma parameters as functions of the variable s=(I/r)2/5(current I, distance r). The main results give a quantitative decomposition of the force accelerating the ions into three partial forces caused by the electric field, by the ion pressure gradient and by the electron-ion friction, which are of comparable importance. Unlike the ions, the electrons are accelerated by the pressure gradient only, but are decelerated both by the field and by friction; all these forces cancel each other almost completely. The direction of the electric current is opposite to the direction of the field; therefore the resistance of the plasma is negative. The limitations of the validity of the derived solution are discussed, as are possible extensions and simplifications of the model.

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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

The theoretical basis of the random walk movement of cathode arc spots is reconsidered, and the probable causes of some discrepancies in measured spot movement parameters and in their interpretation are discussed.