Ultralow Surface Resistance via Vacuum Heat Treatment of Superconducting Radio-Frequency Cavities

We report on an effort to improve the performance of superconducting radiofrequency cavities by using heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is decreased significantly, and we find an unexpected reduction in the Bardeen-Cooper-Schrieffer (BCS) resistance. Together these result in extremely high-quality factor values at relatively large accelerating fields E-acc similar to 20 MV/m: Q(0) = 3-4 x 10(11) at <1.5 K and Q(0) similar to 5 x 10(10) at 2.0 K. In one cavity, measurements of surface resistance versus temperature showed an extremely small residual resistance of just 0.63 +/- 0.06 n Omega at 16 MV/m. Secondary ion mass spectrometry measurements confirm that the oxide is dissociated significantly, but they also show the presence of nitrogen after heat treatment. We also present studies of surface oxidation via exposure to air and to water, as well as the effects of very light surface removal via HF rinse. The study is performed on 1.3 GHz cavities, but the effect may be extendable to other frequencies as well. The possibilities for applications and the planned future development are discussed.