Search for: All records

${\mathrm{Nb}}_3\mathrm{Sn}$film coatings have the potential to drastically improve the accelerating performance of Nb superconducting radiofrequency (SRF) cavities in next-generation linear particle accelerators. Unfortunately, persistent ${\mathrm{Nb}}_3\mathrm{Sn}$stoichiometric material defects formed during fabrication limit the cryogenic operating temperature and accelerating gradient by nucleating magnetic vortices that lead to premature cavity quenching. The SRF community currently lacks a predictive model that can explain the impact of chemical and morphological properties of ${\mathrm{Nb}}_3\mathrm{Sn}$defects on vortex nucleation and maximum accelerating gradients. Both experimental and theoretical studies of the material and superconducting properties of the first 100 nm of ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces are complicated by significant variations in the volume distribution and topography of stoichiometric defects. This work contains a coordinated experimental study with supporting simulations to identify how the observed chemical composition and morphology of certain Sn-rich and Sn-deficient surface defects can impact the SRF performance. ${\mathrm{Nb}}_3\mathrm{Sn}$films were prepared with varying degrees of stoichiometric defects, and the film surface morphologies were characterized. Both Sn-rich and Sn-deficient regions were identified in these samples. For Sn-rich defects, we focus on elemental Sn islands that are partially embedded into the ${\mathrm{Nb}}_3\mathrm{Sn}$film. Using finite element simulations of the time-dependent Ginzburg-Landau equations, we estimate vortex nucleation field thresholds at Sn islands of varying size, geometry, and embedment. We find that these islands can lead to significant SRF performance degradation that could not have been predicted from the ensemble stoichiometry alone. For Sn-deficient ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces, we experimentally identify a periodic nanoscale surface corrugation that likely forms because of extensive Sn loss from the surface. Simulation results show that the surface corrugations contribute to the already substantial drop in the vortex nucleation field of Sn-deficient ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces. This work provides a systematic approach for future studies to further detail the relationship between experimental ${\mathrm{Nb}}_3\mathrm{Sn}$growth conditions, stoichiometric defects, geometry, and vortex nucleation. These findings have technical implications that will help guide improvements to ${\mathrm{Nb}}_3\mathrm{Sn}$fabrication procedures. Our outlined experiment-informed theoretical methods can assist future studies in making additional key insights about ${\mathrm{Nb}}_3\mathrm{Sn}$stoichiometric defects that will help build the next generation of SRF cavities and support related superconducting materials development efforts. Published by the American Physical Society2024 

Abstract Surface properties are critical to the capabilities of superconducting microwave devices. The native oxide of niobium-based devices is thought to consist of a thin normal conducting layer. To improve understanding on the importance of this layer, an attempt was made to replace it with a more easily controlled gold film. A niobium sample host microwave cavity was used to measure the surface resistance in continuous wave operation at $4.0\text{GHz}$and $5.2\text{GHz}$. Sample conditions studied include temperatures ranging from $1.6\text{K}$to $4.2\text{K}$with RF magnetic fields on the sample surface ranging from $\sim 1\text{mT}$to the maximum field before the superconducting properties were lost (quench field). The nominal film thickness of the gold layer was increased from $0.1\text{nm}$to $2.0\text{nm}$in five steps to study the impact of the normal layer thickness on surface resistance on a single niobium substrate. The $0.1\text{nm}$film was found to reduce the surface resistance of the sample and to enhance the quench field. With the exception of the final step from a $1.5\text{nm}$gold film to $2.0\text{nm}$, the magnitude of the surface resistance increased substantially with gold film thickness. The nature of the surface resistance field-dependence appeared to be roughly independent from the gold layer thickness. This initial study provides new perspectives and suggests avenues for optimizing and designing surfaces for resonant cavities in particle accelerators and quantum information applications. 

Abstract Surface structures on radio-frequency (RF) superconductors are crucially important in determining their interaction with the RF field. Here we investigate the surface compositions, structural profiles, and valence distributions of oxides, carbides, and impurities on niobium (Nb) and niobium–tin (Nb₃Sn)in situunder different processing conditions. We establish the underlying mechanisms of vacuum baking and nitrogen processing in Nb and demonstrate that carbide formation induced during high-temperature baking, regardless of gas environment, determines subsequent oxide formation upon air exposure or low-temperature baking, leading to modifications of the electron population profile. Our findings support the combined contribution of surface oxides and second-phase formation to the outcome of ultra-high vacuum baking (oxygen processing) and nitrogen processing. Also, we observe that vapor-diffused Nb₃Sn contains thick metastable oxides, while electrochemically synthesized Nb₃Sn only has a thin oxide layer. Our findings reveal fundamental mechanisms of baking and processing Nb and Nb₃Sn surface structures for high-performance superconducting RF and quantum applications. 

Abstract Workbench-size particle accelerators, enabled by Nb₃Sn-based superconducting radio-frequency (SRF) cavities, hold the potential of driving scientific discovery by offering a widely accessible and affordable source of high-energy electrons and x-rays. Thin-film Nb₃Sn RF superconductors with high quality factors, high operation temperatures, and high-field potentials are critical for these devices. However, surface roughness, non-stoichiometry, and impurities in Nb₃Sn deposited by conventional Sn-vapor diffusion prevent them from reaching their theoretical capabilities. Here we demonstrate a seed-free electrochemical synthesis that pushes the limit of chemical and physical properties in Nb₃Sn. Utilization of electrochemical Sn pre-deposits reduces the roughness of converted Nb₃Sn by five times compared to typical vapor-diffused Nb₃Sn. Quantitative mappings using chemical and atomic probes confirm improved stoichiometry and minimized impurity concentrations in electrochemically synthesized Nb₃Sn. We have successfully applied this Nb₃Sn to the large-scale 1.3 GHz SRF cavity and demonstrated ultra-low BCS surface resistances at multiple operation temperatures, notably lower than vapor-diffused cavities. Our smooth, homogeneous, high-purity Nb₃Sn provides the route toward high efficiency and high fields for SRF applications under helium-free cryogenic operations.