Niobium superconducting radio-frequency (SRF) cavities are widely used accelerating structures. Improvements in both quality factor, Q0, and... Show moreNiobium superconducting radio-frequency (SRF) cavities are widely used accelerating structures. Improvements in both quality factor, Q0, and maximum accelerating gradient, Eacc, have been made to SRF cavities by introducing new processing techniques. These breakthroughs include processes such as nitrogen doping(N-Doping) and infusion, electrochemical polishing (EP) and High Pressure Rinsing
(HPR). [1] There is still abundant opportunity to improve the cavities or, rather, the
material they’re primarily composed of: niobium. A focus here is the role the native
oxide of Nb plays in SRF cavity performance. The values of interest in a given cavity
are its quality factor Q0, maximum accelerating gradient Eacc and surface resistance
Rs
. This work characterizes Nb and Ta foils prepared under identical conditions using
X-ray photoelectron spectroscopy (XPS) to compare surface oxides and better understand RF loss mechanisms in Nb SRF cavities and qubits. It is well established that
Ta qubits experience much longer coherence times than Nb qubits, which is probably
due to the larger RF losses in Nb oxide. By studying Tantalum, an element similar to
Niobium, the mechanisms of the losses that originate in the oxide and suboxide layers
present on the surface of Nb cavities might finally be unlocked. We find noticeable
differences in the oxides of Nb and Ta formed by air exposure of clean foils. In particular, Ta does not display the TaO2 suboxide in XPS, while Nb commonly shows
NbO2. This suggests that suboxides are an additional contributor of RF losses. We
also suggest that thin Ta film coatings of Nb SRF cavities may be a way of increasing
Q0. It is in the interest of the accelerator community to fully understand the surface
impurities present in Nb SRF cavities so that strategies for mitigating the effects can
be proposed. Show less
