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

Abstract

The influence of heat treatments at 122, 400, and $800\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ on the field emission of large-grain and single-crystal high-purity niobium samples has been investigated. Buffered chemical polishing of $40\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ 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 $40/{\mathrm{cm}}^{2}$ with temperature were found at surface fields up to $160\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. 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 $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ 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.