Enhancement in Quality Factor of SRF Niobium Cavities by Material Diffusion

We report the increase in the quality factor of superconducting radiofrequency cavities when small concentrations of nonmagnetic impurities such as titanium and nitrogen are thermally diffused into the inner surface. Several cavities made from ingot niobium with residual resistivity ratio (RRR) $\sim$ 100–300 and a fine-grain reactor grade cavity with RRR $<$ 100 were thermally doped with titanium and nitrogen in the temperature range of 800 $^{\circ}\hbox{C}$–1400 $^{\circ}\hbox{C}$. The magnitude of the increase in quality factor is related to the process parameters and is similar to that observed in fine-grain high-purity niobium cavities.