Analysis of Nb3Sn surface layers for superconducting radio frequency cavity applications

Abstract

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 ...

Performance-defining properties of Nb3Sn coating in SRF cavities

“…Smaller lattice constants most likely originate from the Sn-deficiency regions observed by STEM/EDS. The presence of a spread in lattice constants was previously reported in Nb 3 Sn coating prepared by the same method [20].…”

“…The effect of local compositional inhomogeneity on T c for A15 compounds was first realized in the 1980s from specific heat and tunneling studies [18,19]. In [20], tin-deficient regions within the Nb 3 Sn film are reported in a witness sample coated with single cell cavity ERL1-4 which, as shown in figure 1, did not exhibit strong Q-slope, but did quench. There is no evidence that these regions are the cause of the quench degradation observed regularly between 14 and 17 MV m −1 in Nb 3 Sn cavities, but they are a candidate, based on the expected reduction in superconducting properties in these regions.…”

Radio Frequency Magnetic Field Limits of Nb andNb3Sn

“…The dark and light shaded bands show the approximate temperature dependence of H sh and H c1 extracted from CW measurements. regions near the surface of a witness sample coated with the cavity [44].…”

“…Calculations based on extracted material parameters indicate that H c1 of Nb 3 Sn is not a fundamentally limiting field. Future Nb 3 Sn research will focus on optimizing parameters used in the coating process in order to reduce the presence of defects in the material, with a focus on low tin content regions that have been observed near the surface [44]. …”

Photocathode quantum efficiency of ultrathin Cs2Te layers on Nb substrates

“…A replaceable, superconducting plug cathode would be particularly attractive for a compact SRF linac as it has a simple design [2,3]. Recent advances in Nbbased SRF cavities, including record high Q values at 15-20 MV=m via a nitrogen doping process [4], as well as the successful in situ growth of higher T C Nb 3 Sn on the inside surface [5,6] suggests that a compact electron linac operating at 4.2 K is feasible in the future. A limiting factor is that the present choices of superconducting photocathode have relatively low quantum efficiencies (QE), e.g., for Nb QE < 0.01% and for Pb QE < 0.1% at 248 nm wavelength [3].…”