Field emitter activation on cleaned crystalline niobium surfaces relevant for superconducting rf technology

The influence of heat treatments at 122, 400, and 800°C on the field emission of large-grain and single-crystal high-purity niobium samples has been investigated. Buffered chemical polishing of 40 μm and high pressure ultrapure water rinsing under clean-room conditions resulted in smooth surfaces with a linear surface roughness of 46 to 337 nm. By means of field emission scanning microscopy, an increasing number of emitters up to 40/cm^{2} with temperature were found at surface fields up to 160 MV/m. Two different mechanisms of emitter activation were found, i.e. activation by the applied electric field and activation by temperature. Some emitters with an onset surface field of 50 to 100 MV/m appeared already after the low-temperature bakeout. Correlated scanning-electron-microscopy/energy-dispersive-x-ray measurements revealed particles and surface defects as emitters. Their activation will be discussed with respect to the thickness of the insulating oxide layer