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

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Abstract
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 obtained surfaces of cm2 size which do not emit at 100 MV/m. If such an emission-free surface is heat treated again at 800 °C, new emitters appear. Typically, the physical size of the field emitters is a few μm, although in some cases a larger particle was found and in others no feature was seen at 0.5-μm resolution. A large variety of elements is associated with these particles. We discuss in particular the nature and origin of three types of emitters: (1) sulfur containing particles, (2) carbon particles, both of which were identified as new emitters after a 800 °C HT, and (3) FE sites which are especially resistant against HT.