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 cathode materials 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 cathode materials 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%.

Cathode properties of arcs are investigated for ambient gas pressures of 10−3−725 Torr. Copper, silver, and carbon electrodes are used in controlled ambients of either nitrogen, argon, or helium. Experimental techniques include pressure-dependent observations of the ionized cathode vapor detected at cylindrical collectors surrounding the electrodes, arc photography, and measurements of the cathode erosion rate for copper at 100 A using the weight loss method. At pressures approaching atmospheric, it is shown that the measured erosion rate is reduced from that in vacuum by about an order of magnitude. Increasing the ambient pressure also decreases the distance from the cathode spots at which the 8% ion current can be detected. Data extrapolation shows that cathode spots at atmospheric pressure, particularly in the lighter gases, are still associated with the fundamental ion current. Comparison between the measured reductions in both the erosion rates and the ion currents indicates that vapor and ion redeposition on the cathode contribute to the decreased erosion. It is concluded that the cathode spots of low current arcs at pressures approaching atmospheric are basically similar to vacuum arc cathode spots. The final section of the paper compares current-dependent erosion data for copper arcs in vacuum and air, and shows the importance of collective cathode spot heating.

No abstract

No abstract

No abstract