Field Emission from Niobium in the Normal and Superconducting States

The change of field emission current from a superconductor as a result of a transition to the normal state under constant field and temperature conditions was calculated. The derivation is based on the Bardeen-Cooper-Schrieffer formulation of the density of states and band gap model for a superconductor. The experimental method for determining the current change consisted in measuring the current increment between a niobium field emitter in the superconducting state at 4.2°K and in the normal state at a temperature $T$ above the transition. After correcting for the current increase occurring as a result of the temperature increase alone, the residual would be that associated with the superconducting-normal transition. This was less than the detectability of the measurements, although the calculations indicated an expected increase in current easily measurable if the superconducting-normal transition had occurred at the emitter surface. It is speculated that under high-field conditions, the superconducting state is quenched at the surface.