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.
Highlights
Field emission (FE) is one of the main limitations for the achievable accelerating gradient Eacc of superconducting (SC) niobium cavities required for the European x-ray free electron laser (XFEL) [1] and the planned International Linear Collider (ILC) [2] leading to dark currents, x-ray load, radiation activation, power losses, etc
Advanced surface preparation and clean-room techniques for the high-purity Nb cavities for the superconducting radiofrequency (SRF) technology [3] have to be applied to prevent FE at electric surface fields Epeak up to 50 MV=m for the XFEL and up to 70 MV=m for the ILC in case of TESLAtype cavities [4] or even up to 85 MV=m if low loss shape [5] of the cavities will be chosen
The sequential field emission scanning microscope (FESM)-heat treatments (HTs)-FESM investigations led to the expected activation of emitters with temperature
Summary
Field emission (FE) is one of the main limitations for the achievable accelerating gradient Eacc of superconducting (SC) niobium cavities required for the European x-ray free electron laser (XFEL) [1] and the planned International Linear Collider (ILC) [2] leading to dark currents, x-ray load, radiation activation, power losses, etc. Advanced surface preparation and clean-room techniques for the high-purity Nb cavities for the superconducting radiofrequency (SRF) technology [3] have to be applied to prevent FE at electric surface fields Epeak up to 50 MV=m for the XFEL and up to 70 MV=m for the ILC in case of TESLAtype cavities [4] or even up to 85 MV=m if low loss shape [5] of the cavities will be chosen. First experiments with a single large-grain (LG) highpurity Nb sample [16], which have been 100 m BCP etched and 150 bar HPR treated have supported this idea for some emitters but at rather high fields of 250 and 300 MV=m activated by a bakeout at 150C for 14 hours. A systematic investigation of the influence of the different HTs on the FE of SC and LG Nb samples prepared with the actual techniques at DESY has been started
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