Effect of low temperature baking in nitrogen on the performance of a niobium superconducting radio frequency cavity

We report the rf performance of a single cell superconducting radiofrequency cavity after low temperature baking in a nitrogen environment. A significant increase in quality factor has been observed when the cavity was heat treated in the temperature range of 120--160 ^∘C with a nitrogen partial pressure of ~25 m Torr. This increase in quality factor as well as the Q-rise phenomenon (anti-Q-slope) is similar to those previously obtained with high temperature nitrogen doping as well as titanium doping. In this study, a cavity N₂-treated at 120 ^∘C and at 140 ^∘C showed no degradation in accelerating gradient, however the accelerating gradient was reduced by ~25% with a 160 ^∘C N₂ treatment, compared to the baseline tests after electropolishing. Sample coupons treated in the same conditions as the cavity were analyzed by scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a complex surface composition of Nb₂O₅, NbO and NbN_(1-x)Oₓ within the rf penetration depth. Furthermore, magnetization measurements showed no significant change on bulk superconducting properties.