Vacuum Breakdown between Superconducting Electrodes

Abstract

IN this communication I report the preliminary results of a study of prebreakdown current-voltage characteristics of superconducting electrodes in a high vacuum. The small prebreakdown electron current which flows from the cathode when a sufficiently high potential difference is applied between two electrodes in a vacuum is described quantitatively by the Fowler–Nordheim equation if it is assumed that the applied electric field is enhanced by a factor β due to small projections on the cathode surface^1–3. Gomer and Hulm⁴ have measured currents from tantalum electrodes both in the normal and superconducting states. They could detect no difference in the current, I, at temperatures below and above the transition temperature when an electric field Ewas applied.The results expressed in the form of a Fowler–Nordheim graph of against yielded straight lines throughout the temperature range 2.2 K to 270 K. By contrast, Looms et al.⁵ used niobium at a pressure of 10⁻⁵ torr and showed that as the temperature of the electrodes decreased from 300 K to 4.2 K, the current, at constant applied field, decreased by several orders of magnitude although no discontinuity was observed at the onset of superconductivity. Straight Fowler–Nordheim graphs were not obtained and they concluded that the data were not consistent with the Fowler–Nordheim relationship in this temperature range.