Development and Understanding of Nb₃Sn films for radiofrequency applications through a sample-host 9-cell cavity

Abstract

Nb3Sn is a promising advanced material under development for superconducting radiofrequency (SRF) cavities. Past efforts have been focused primarily on small development-scale cavities, but large, often multi-celled cavities, are needed for particle accelerator applications. In this work, we report on successful Nb3Sn coatings on Nb in a 1 m-long 9-cell Nb sample-host cavity at Fermilab. The geometry of the first coating with only one Sn source made it possible to study the influence of Sn flux on the microstructure. Based on these results, we postulate a connection between recently observed anomalously large thin grains and uncovered niobium spots observed in the past by other authors [Trenikhina 2018]. A phenomenological model to explain how these anomalously large grains could form is proposed. This model is invoked to provide possible explanations for literature results from several groups and to guide key process parameters to achieve uniform vapor-diffusion coatings, when applied to complex structures as the multi-cell cavity under study. Using the current methods that achieve strong performance on single-cell cavities and scaling them up to multicell cavities is not necessarily straightforward due to the nature of the process, which involves thermal evaporation of Sn vapor from a heated source. Due to the complex geometry of SRF cavities, substrate surfaces occupy a variety of angles and distances from the source, as well as areas outside of the line-of-sight, issues which have complicated other coating processes as well (e.g. Nb/Cu).

Advances in Nb3Sn superconducting radiofrequency cavities towards first practical accelerator applications

“…Steps 2 and 3 to maintain high vapor pressure were recommended by Hillenbrand et al [22] and a proposed explanation for the benefit provided by the high vapor pressure is reducing the formation of anomalously large thin grains [18].…”

“…Visual evaluation would often give useful indications of coating quality. Examples include patterns of dark spots (interpreted as tin condensation) [18], local shiny spots (interpreted as anomalously large thin grains), and a shiny, almost glossy appearance over the whole surface (interpreted as a low surface roughness coating).…”

“…This step is believed to increase the density of Sn nucleation sites on the surface, especially in regions that do not have line of sight to the coating source. This in turn is expected to reduce the formation of "patchy" regions on the surface [18]. The high density of nucleation sites may also contribute to the smaller grain size in the final film.…”

Development and characterization of Nb3Sn/Al2O3 superconducting multilayers for particle accelerators

“…The significant local non-stoichiometry of thick (a few micron) polycrystalline Nb 3 Sn coatings of Nb cavities 20 , 27 , as well as Sn depletion at grain boundaries in Nb 3 Sn 34 – 37 have been well documented in the literature. Yet, despite these materials issues which are also characteristic of 1–3 μm thick Nb 3 Sn films used in SRF cavities 38 , our Nb 3 Sn SIS structures exhibit higher low-field Q values than Nb at T > 6 K 20 , consistent with the larger superconducting energy gap and a lower BCS surface resistance of Nb 3 Sn. These experimental results not only show a remarkable resilience of low-field quality factors of Nb 3 Sn to the significant non-stoichiometry and materials imperfections but also suggest that the SRF performance of Nb 3 Sn coatings can be further improved by materials treatments.…”

Development and Characterization of Nb3Sn/Al2O3 Superconducting Multilayers for High-performance Radio-Frequency Applications

“…The significant local non-stoichiometry of thick (a few micron) polycrystalline Nb3Sn coatings of Nb cavities 20,27 , as well as Sn depletion at grain boundaries in Nb3Sn [34][35][36][37] have been well documented in the literature. Yet, despite these materials issues which are also characteristic of 1-3 m thick Nb3Sn films used in SRF cavities 38 , our Nb3Sn SIS structures exhibit higher low-field Q values than Nb at T > 6K 20 , consistent with the larger superconducting energy gap Δ Nb3Sn ≈ 2Δ Nb and a lower BCS surface resistance ∝ low-field quality factors of Nb3Sn to the significant non-stoichiometry and materials imperfections but also suggest that the SRF performance of Nb3Sn coatings can be further improved by materials treatments. Our Nb3Sn multilayers exhibit a similar resilience of the low-power SRF performance to the materials imperfections.…”