Field emission from broad‐area niobium cathodes: Effects of high‐temperature treatment

We present results made with a new apparatus for the study of localized field emission (FE) sites on broad‐area cathodes. The apparatus is centered around a UHV field emission scanning microscope, consisting of a micromanipulator allowing precision cathode motion, a rotatable holder for broad and microtip anodes, and a fast high‐voltage regulator allowing constant‐current measurement of a distribution of both weakly and strongly emitting sites. Also included are an in situ scanning electron microscope, and a facility for in situ microfocus Auger analysis as well as scanning Auger microscopy. Samples can be high‐temperature annealed without removal from UHV. These tools have been used to study the effects of heat treatment (HT) up to 2000 °C on the FE from nonanodized and anodized Nb cathodes. We find that HT at 800–900 °C increases the density of field emitters at a given electric field. HT at T>1000 °C reduces the density, while at T≥1400 °C a drastic decrease of the emission occurs. We have repeatedly o...