We have examined the particles emitted radially by dc arcs drawn in a vacuum ambient on cathodes of several elements, as well as the axial (through-anode) ion flux from a copper cathode. The axial results for copper agree grossly with the radial copper results. Significant quantities of multiply charged ions were seen for all elements examined. All arc volt-ampere characteristics were positive in the range of currents observed: 30 to 250 A. We drew the following conclusions (normalizing energy in units of ion energy/ion charge): (1) The energy distributions for the various ions are similar, peaking at potentials well above the arc voltage. (2) The fraction of ions that are singly charged tends to increase with increasing arc current. (3) For a given element, as the degree of ionization increases the location of the ion-energy distribution peak shifts to lower energies. (4) For a given degree of ionization, the location of the peak tends toward higher energies for elements with greater arc voltages. (5) The location of the peak shifts toward lower energies as the arc current increases. A possible mechanism for the production of ions with energies corresponding to potentials greater than the anode potential lies in the theory of a potential peak near the cathode.

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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. The authors' calculations are based mainly upon the energy distribution data of the different ion fluxes, the data available for various cathode materials are summarized. They 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. They 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 reasonable well with the experimental data. The ratios vp/v0 (vp is the most probable velocity) and v0/ beta are evaluated and used in discussing theoretical ion acceleration mechanisms. Their analysis suggests that the mechanism making the greatest contribution to the energy of the ion flux is the electro-ion collision mechanism, but that the contributions of the electric field force and flow interaction (pressure gradient) mechanisms are also significant. Their 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.

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