Field of first magnetic flux entry and pinning strength of superconductors for rf application measured with muon spin rotation

Abstract

The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry Hentry and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation (µSR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400 • C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure Hentry of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.Recently, to reach high quality factors, a treatment procedure has been established baking cavities at 800 • C and injecting nitrogen gas at the end of this treatment. arXiv:1705.05480v3 [cond-mat.supr-con]

Superheating in coated niobium

“…It is therefore beneficial to use a DC method to measure H entry . For this purpose we have established a muon spin rotation (µSR) experiment [8]. Spin polarized muons with an average stopping distance of 130 µm are implanted one at a time into the sample.…”

“…All samples were made from fine grain niobium from Tokyo Denkai with a residual resistance ratio (RRR) above 300. In [8] a detailed study of geometry and pinning has been carried out. Depending on sample and field geometry, H entry measurements can potentially give an artificially larger value due to flux pinning.…”

“…Here the field is applied in the radial direction. In [8] it has been shown that this geometry is only slightly more sensitive to pinning than the ellipsoidal geometry. For comparison Nb and Nb 3 Sn samples of a coin shape have also been produced for this study.…”

“…Afterwards all samples, except the ones which were used for Nb 3 Sn coating, have received a 1400 • C annealing at TRIUMF. This treatment is effective in releasing virtually all pinning [8]. After the heat treatment the samples received an additional BCP to remove another 30 µm of material.…”

Critical fields of Nb3Sn prepared for superconducting cavities

“…The local magnetic field sensed in this case is from the nuclear dipolar fields. In case of significant muon diffusion, which is the case for high RRR niobium [14], the polarization is described by the dynamic zero field Kubo-Tuyabe function P dyn ZF [20] . Therefore, in the field of first vortex penetration measurements P dyn ZF will be sensed for low applied field.…”