A microns-thick film of Nb3Sn on the inner surface of a superconducting radiofrequency (SRF) cavity has been demonstrated to substantially improve cryogenic efficiency compared to the standard niobium material, and its predicted superheating field is approximately twice as high. We review in detail the advantages of Nb3Sn coatings for SRF cavities. We describe the vapor diffusion process used to fabricate this material in the most successful experiments, and we compare the differences in the process used at different labs. We overview results of Nb3Sn SRF coatings, including CW and pulsed measurements of cavities as well as microscopic measurements. We discuss special considerations that must be practised when using Nb3Sn cavities in applications. Finally, we conclude by summarizing the state-of-the-art and describing the outlook for this alternative SRF material.

We report on atomic-scale analyses of the microstructure of an Nb 3 Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and first-principles calculations.Epitaxial growth of Nb 3 Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb 3 Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb 3 Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb 3 Sn (12 ̅ 0)//Nb (1 ̅ 11) and Nb 3 Sn (002)//Nb (01 ̅ 1). The Nb 3 Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb (01 ̅ 1) and Nb 3 Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb 3 Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb 3 Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb 3 Sn/Nb interface provides important information about the formation of imperfections in Nb 3 Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb 3 Sn SRF cavities for accelerators.

We present an analysis of Nb 3 Sn surface layers grown on a bulk Niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb 3 Sn coatings on 1.3 GHz Nb cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 ±0.4 meV and superconducting critical temperatures (T c ) up to 16.3K. Scanning transmission electron microscopy (STEM) performed on cross sections of the sample's surface region shows a ∼ 2 microns thick Nb 3 Sn surface layer. The elemental composition map exhibits a Nb:Sn ratio of 3:1 and reveals the presence of buried sub-stoichiometric regions that have a ratio of 5:1. Synchrotron x-ray diffraction experiments indicate a polycrystalline Nb 3 Sn film and confirm the presence of Nb rich regions that occupy about a third of the coating volume. These low T c regions could play an important role in the dissipation mechanisms occurring during RF tests of Nb 3 Sn -coated Nb cavities and open the way for further improving a very promising alternative to pure Nb cavities for particle accelerators.Discovered in 1954 1 , the A-15 compound Nb 3 Sn is a Type II (κ∼20) strong coupling s-wave superconductor 2,3 with a maximum T c of 18 K 4 and superconducting order parameter ∆ of 3.4 meV 5 . Due to its relatively high T c and ability to carry high current densities, Nb 3 Sn is an ideal candidate for replacing NbTi for superconducting wire applications and Nb for superconducting radio frequency (SRF) resonators operating from a few hundred MHz up to several GHz. Early work into developing Nb 3 Sn for SRF applications started in the 1970s 6-9 . In particular, researchers from Wuppertal University optimized a coating recipe 8 based on the diffusion of Sn vapor into elemental Nb at temperatures between 1000 • C to 1200 • C. This approach has the unique advantage of being scalable to applications for which a coating process without a direct line of sight is required. State-of-the-art RF performance tests then 10 showed an extremely high quality factor ∼ 10 11 at 2K and ∼ 10 10 at 4.2K (about 20 times higher than pure Nb) with a strong decrease of the quality factor (Q -slope) above an accelerating field of 5 MV/m. The origin of this Q -slope remains unclear, however it was postulated that the onset of the Q decrease at 5 MV/m (peak surface magnetic field of 22 mT) was due to early vortex penetration above the Nb 3 Sn first critical field B C1 , and therefore was an intrinsic material limitation. A regain of interest was stimulated by recent RF tests done at Cornell University 11 that reproducibly exhibit a similar Q factor ∼ 2 × 10 10 at 4.2K (and 3 × 10 10 at 2K), but a very moderate Q -slope up to a quenching field of 12-17 MV/m, corresponding to a a) Electronic mail: prolier@anl.gov peak surface magnetic field of 50-70 mT, which is significantly higher than the B C1 of 25±7 mT measured on this cavity 11 . Another striking and reproducible feature is the very moderate ...

Many future particle accelerators require hundreds of superconducting radiofrequency (SRF) cavities operating with high duty factor. The large dynamic heat load of the cavities causes the cryogenic plant to make up a significant part of the overall cost of the facility. This contribution can be reduced by replacing standard niobium cavities with ones coated with a low-dissipation superconductor such as Nb3Sn. In this paper, we present results for single cell cavities coated with Nb3Sn at Cornell. Five coatings were carried out, showing that at 4.2 K, high Q0 out to medium fields was reproducible, resulting in an average quench field of 14 MV/m and an average 4.2 K Q0 at quench of 8 × 109. In each case, the peak surface magnetic field at quench was well above Hc1, showing that it is not a limiting field in these cavities. The coating with the best performance had a quench field of 17 MV/m, exceeding gradient requirements for state-of-the-art high duty factor SRF accelerators. It is also shown that—taking into account the thermodynamic efficiency of the cryogenic plant—the 4.2 K Q0 values obtained meet the AC power consumption requirements of state-of-the-art high duty factor accelerators, making this a proof-of-principle demonstration for Nb3Sn cavities in future applications.

The superconducting RF linac for LCLS-II calls for 1.3 GHz 9-cell cavities with an average intrinsic quality factor Q0 of 2.7×10 10 at 2.0 K and 16 MV/m accelerating gradient. Two niobium 9 cell cavities, prepared with nitrogen-doping at Fermilab, were assembled into the Cornell Horizontal Test Cryomodule (HTC) to test cavity performance in a cryomodule that is very similar to a full LCLS-II cryomodule. The cavities met LCLS-II specifications with an average quench field of 17 MV/m and an average Q0 of 3×10 10 . The sensitivity of the cavities' residual resistance to ambient magnetic field was determined to be 0.5 nΩ/mG during fast cool down. In two cool downs, a heater attached to one of the cavity beam tubes was used to induce large horizontal temperature gradients.Here we report on the results of these first tests of nitrogen-doped cavities in a cryomodule, which provide critical information for the LCLS-II project.