WEIAA01
LSU, Baton Rouge, USA
Superconducting RF (SRF) cavity resonators with ultrahigh-Q, originally developed for particle accelerator technology, are a key technology platform for detectors of rare events, e.g. light by light scattering mediated by virtual electron-positron pairs, axions [1] and high-frequency gravitational waves [2]. The mechanism(s) leading to current limits in Q are not fully understood. We developed a numerical method to calculate Q and cavity resonant frequency shifts based on nonequilibrium theory of superconductivity, including the role of impurity disorder, combined with Slater’s method for solving Maxwell’s equations for the EM field confined in a cavity [3]. Our results for the frequency shift and Q are in excellent agreement with experimental data reported by the SRF group at Fermilab [4]. As a measure of the predictive capability of the theory we are able to quantitatively account for changes in the resonant frequency of order 10 Hz for GHz SRF cavities over temperature ranges of 0.001 Tc. This level of predictive theory is essential for further improvements in performance of superconducting resonators and devices for quantum sensing and quantum processors.
[1] Z. Bogorad et al., Phys. Rev. Lett. 123, 021801 (2019).
[2] A. Berlin et al., arXiv:2303.0151.
[3] H. Ueki, M. Zarea, and J. A. Sauls, arXiv:2207.14236.
[4] D. Bafia et al., arXiv:2103.10601.
WEIAA02
PKU, Beijing, People’s Republic of China
JLab, Newport News, Virginia, USA
Nb₃Sn offers the prospect of better RF performance (Q and Eacc) than niobium at any given temperature because of its superior superconducting properties. Nb₃Sn-coated SRF cavities are routinely produced by growing a few microns thick Nb₃Sn film on Nb cavities via tin vapor diffusion. It has been observed that a clean and smooth surface can enhance the performance of the Nb₃Sn-coated cavity, typically, the attainable acceleration gradient. The reduction of surface roughness is often linked with a correlative reduction in average coating thickness and grain size. Besides Sn supply’s careful tuning, the temperature profiles were varied to reduce the surface roughness as low as ~40 nm in 20 µm × 20 µm AFM scans, one-third that of the typical coating. Samples were systematically coated inside a mock single-cell cavity and examined using different material characterization techniques. A few sets of coating parameters were used to coat 1.3 GHz single-cell cavities to understand the effects of roughness variation on the RF performance. This presentation will discuss ways to reduce surface roughness with results from a systematic analysis of the samples and Nb₃Sn-coated single-cell cavities.
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
Cornell University, Ithaca, New York, USA
Niobium-3 tin is a promising material for next-generation superconducting RF cavities due to its high critical temperature and high theoretical field limit. There is currently significant worldwide effort aiming to improve Nb₃Sn growth to push this material to its ultimate performance limits. This talk will present an overview of Nb₃Sn cavity development at Cornell University. One approach we are pursuing is to further advance the vapor diffusion process through optimized nucleation and film thickness. Additionally, we are exploring alternative Nb₃Sn growth methods, such as the development of a plasma-enhanced chemical vapor deposition (CVD) system, as well as Nb₃Sn growth via electrochemical synthesis.