A SIMPLE-MODEL OF DIFFUSE VACUUM-ARC PLASMAS

The diffuse expanding plasma of low-current vacuum arcs is composed of and characterized by an almost thermal electron gas and an ion beam with high kinetic energy. It is described theoretically with a two-fluid hydrodynamic model. By application of through simplifications (such as isothermal conditions, constant composition, one-dimensional treatment) an analytical solution is found in the form of asymptotic power series of the plasma parameters. This solution is used to analyse the plasma ion acceleration, resulting in the statement that the three forces acting on ions (i.e. the electric field, the ion pressure gradient, the electron-ion friction) are of comparable importance, contributing to the total ion energy roughly 20%, 35%, and 45%, respectively (in case of Cu vapour arcs). As to the electric field and Ohm's law the solution reveals that the potential is falling towards the anode and that the diffusion current exceeds the opposite conduction current with the consequence of a negative resistance of the arc plasma. Finally, the approximations of the solution, limitations of the validity and possible extensions of the model are shortly discussed.