Erosion and ionization in the cathode spot regions of vacuum arcs

Abstract

The net erosion rate at the cathode spots of 100‐A vacuum arcs has been determined experimentally for Cd, Zn,Ag,Cu, Cr, Fe, Ti, C, Mo, and W electrodes. Ion currents to the metal walls surrounding each of these cathodematerials have also been investigated. For each material, the dependence of the wall ion current on the electrode spacing and anode geometry is consistent with an arc model which assumes predominant vapor ionization in the cathode regions, with subsequent isotropic free flight motion from these regions. Comparison of the net erosion rate with the wall ion current indicates that, for high‐vapor‐pressure materials such as Cd and Zn, ≈ 15% of the vapor leaves the cathode regions ionized. For low‐vapor‐pressure materials such as C, Mo, and W, this fractional ionization is almost 100%. The ion current magnitudes observed at long electrode spacings are similar for each material, and lie in the range 7–10% of the arc current. Ion currents of this magnitude are also predicted for Mg, Al, and Ni using experimental data by Plyutto, Ryzhkov, and Kapin. Since these 13 materials span a wide range of thermal characteristics (from Cd: bp 1038 °K to the refractory materials C, Mo, and W: bp 5973 °K), it is concluded that ion currents of this magnitude can be expected from cathodematerials in general. It is also shown that the erosion rate for nonrefractory electrodes can be reasonably estimated using Ecker's theoretical values for cathode spot current densities and temperatures. Furthermore, this comparison between experiment and theory indicates that the total probability for vapor ionization in the cathode region exceeds 50%.