Reduced field emission of niobium and copper cathodes

Abstract

We have investigated the enhanced field emission of broad-area niobium and copper cathodes by means of an UHV field emission scanning microscope which allows the localization and characterization of individual electron emitting sites down to submicron size. For the first time, ex situ prepared samples of cm2 size stayed field emission free up to 150 MV/m for both high-purity Nb and OFHC copper. Only weak electron emission from a few sites with field enhancement factors of less than 30 was observed at 200 MV/m. This progress results from an advanced preparation technique based on ultrapure water rinsing inside a clean room which reduces any kind of surface contamination. Alternatively, the enhanced field emission from microscopic particles and other surface irregularities on Nb can be much suppressed by in situ UHV annealing at 1400 °C followed by a fast cool down. However, many new strong emitters appear on such surfaces after moderate heating at temperatures around 400 °C. These observations support the crucial role of the surface segregation of interstitial atoms for the enhanced field emission of Nb samples.