INTEGRATED ION FLUX EMITTED FROM THE CATHODE SPOT REGION OF A DIFFUSE VACUUM-ARC

This paper reviews the properties of the cathode ion flux generated in the vacuum arc, concentrating on the characteristics of the ion energy distributions of the cathode ions. The cathode ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply-charged ions. Our calculations are based mainly upon the energy distribution data of the different ion fluxes, the data available for various cathode materials are summarized. We assume that the integrated energy distribution (IED) of the ion flux for a particular material may be treated as the sum of all the fractional ion energy distributions and present plots of such IEDS. We further assume that a shifted Maxwellian type velocity distribution gives the best fit to the IED, and calculate the characteristic parameters (shifted velocity V0, thermal velocity beta and normalization constant C1) for each material. IEDS calculated using these parameters agree reasoable well with the experimental data. The ratios V(p)/V0 (V(p) is the most probable velocity) and V0/beta are evaluated and used in discussing theoretical ion acceleration mechanisms. Our analysis suggests that the mechanism making the greatest contribution to the energy of the ion flux is the electron-ion collision mechanism, but that the contributions of the electric field force and flow interaction (pressure gradient) mechanisms are also significant. Our results should be applicable to any vacuum arc where the cathode spots may be treated as individual emitting sites, i.e. where collective interactions between cathode spots may be neglected.