Low level electron field emission current intensities obtained from niobium samples

We present results obtained with an experimental set-up allowing measurements of DC field emission currents in the range of 10−18 to 10−5 A. The experiments are carried out in ultra high vacuum (P < 10−10 Torr) and at room temperature. The detection of currents in the range of 10−18–10−12 A is performed with an electron multiplier located behind a grid anode. Higher currents are measured using a bulk anode and a picoammeter. The whole system has been designed to study niobium samples from sheets used to build superconducting accelerator cavities. Measures in the low current range have shown electron emission regimes that seem not to fit the Fowler-Nordheim law. A possible mechanism for such emissions involving the surface oxide layer is proposed. Comparisons of the electron field emission from samples prepared by different surface treatments as chemical etching, electropolishing, oxide coating by anodization, are presented.