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If Nb 3 Sn cavities can reach H sh , this would correspond to a gradient of nearly 100 MV/m, which would be extremely beneficial for high energy accelerators. This field has not yet been reached, and experiments suggest that defects in the film are the limitation. Improvements in film quality has led to steady progress in maximum gradient. In 2019, a Fermilab cavity reached a new record gradient of 24 MV/m. This is already useful in many applications. For example, this exceeds the operating gradient requirements of LCLS-II and PIP-II cavities. Left: Improvement in maximum accelerating gradient of Nb3Sn cavities over time. Fermilab R&D recently led to reaching 24 MV/m in a single cell 1.3 GHz cavities. Right: Microscopic images of a Nb 3 Sn film from above (a) and cross section (b).

The SIS structure-a thin superconducting film on a bulk superconductor separated by a thin insulating film-was propsed as a method to protect alternative SRF materials from flux penetration by enhancing the first critical field B c1 . In this work, we show that in fact B c1 = 0 for a SIS structure. We calculate the superheating field B sh , and we show that it can be enhanced slightly using the SIS structure, but only for a small range of film thicknesses and only if the film and the bulk are different materials. We also show that using a multilayer instead of a single thick layer is detrimental, as this decreases B sh of the film. We calculate the dissipation due to vortex penetration above the B sh of the film, and find that it is unmanageable for SRF applications. However, we find that if a gradient in the phase of the order parameter is introduced, SIS structures may be able to shield large DC and low frequency fields. We argue that the SIS structure is not beneficial for SRF cavities, but due to recent experiments showing low-surface-resistance performance above B c1 in cavities made of superconductors with small coherence lengths, we argue that enhancement of B c1 is not necessary, and that bulk films of alternative materials show great promise.

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).

The International Linear Collider (ILC) is on the table now as a new global energyfrontier accelerator laboratory taking data in the 2030's. The ILC addresses key questions for our current understanding of particle physics. It is based on a proven accelerator technology. Its experiments will challenge the Standard Model of particle physics and will provide a new window to look beyond it. This document brings the story of the ILC up to date, emphasizing its strong physics motivation, its readiness for construction, and the opportunity it presents to the US and the global particle physics community.