Smooth, homogeneous, high-purity Nb₃Sn superconducting RF resonant cavity by seed-free electrochemical synthesis

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.

Surface oxides, carbides, and impurities on RF superconducting Nb and Nb3Sn: a comprehensive analysis

“…Approximately 2 µm thick Nb 3 Sn films were produced using two methods: (i) Sn-vapor diffusion [16] and (ii) a recently developed process that involves combining electrochemical Sn deposits with thermal conversion to a smoother Nb 3 Sn [17]. The average surface roughnesses (R a ) for these two methods were 300 nm and 40 nm, respectively.…”

“…Characterizing surface oxides on Nb 3 Sn is crucial due to the current challenges faced by Nb 3 Sn cavities, which are hindered by unclear quench mechanisms [16,17]. The theory surrounding how surface oxides influence Nb 3 Sn cavities is evolving [18,19].…”

“…With claims of new recipes and new associated physics emerging, there is a need to provide an accurate picture of the processed Nb surface to build the foundation used for relevant theoretical modeling and process development. Secondary ion mass spectroscopy results have been widely reported [11,17,42,44,46]. Confusion abounds concerning the extremely low nitrogen concentration (∼0.01 at.%) in 'N 2 -infused' Nb, in contrast to 1-3 orders of magnitude higher oxygen and carbon concentrations (figure S1) [46].…”

“…The normalconducting design could potentially utilize oxides of the desired thickness, structure, and electrical properties [13]. One of the objectives of this work is to provide a map of the structures and depths of surface oxides for (i) in situ heated Nb samples under ultra-high vacuum (UHV) and nitrogen (N 2 ) environments, (ii) 13 types of Nb coupon samples processed by acid/polishing, ozone, anodization, and heat treatments covering nearly all the protocols used in SRF preparations, (iii) Nb 3 Sn coupons made by conventional vapor diffusion [16] and the new electrochemical synthesis [17], and their in situ vacuum heated samples.…”

ZrNb(CO) RF Superconducting Thin Film with High Critical Temperature in the Theoretical Limit

“…[ 29 ] Still, current Nb 3 Sn cavities suffer from the typical quench fields of 13 – 18 MV m ‐1 E acc (with the highest value of 24 MV m ‐1 [ 29 ] ), considerably lower than the predicted ultimate limit of ≈ 100 MV m ‐1 . [ 29 ] While ongoing atomic [ 30 ] and theoretical [ 31 ] analyses combined with the growth optimization of Nb 3 Sn films [ 32 ] continue, further efforts are called for to seek alternative materials and surface structures.…”