Ultra-Low Surface Resistance via Vacuum Heat Treatment of Superconducting Radiofrequency Cavities

TL;DR

This paper demonstrates that vacuum heat treatment of superconducting radiofrequency cavities significantly reduces surface resistance, leading to higher quality factors and potential for improved accelerator performance.

Contribution

It introduces a novel heat treatment process that dissociates surface oxide and reduces residual and BCS resistance in superconducting cavities, enhancing their performance.

Findings

Residual resistance decreased significantly after heat treatment

High quality factors achieved at large accelerating fields

Surface oxide dissociation confirmed by SIMS measurements

Abstract

We report on an effort to improve the performance of superconducting radiofrequency cavities by the use of heat treatment in a temperature range sufficient to dissociate the natural surface oxide. We find that the residual resistance is significantly decreased, and we find an unexpected reduction in the BCS resistance. Together these result in extremely high quality factor values at relatively large accelerating fields Eacc ~20 MV/m: Q0 of 3-4x10^11 at <1.5 K and Q0 ~5x10^10 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 nOhms at 16 MV/m. SIMS measurements confirm that the oxide was significantly dissociated, 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 possibilities for applications and the planned future development are discussed.