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]

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Nb3Sn is currently the most promising material other than niobium for future superconducting radiofrequency cavities. Critical fields above 120 mT in pulsed operation and about 80 mT in CW have been achieved in cavity tests. This is large compared to the lower critical field as derived from the London penetration depth, extracted from low field surface impedance measurements. In this paper direct measurements of the London penetration depth from which the lower critical field and the superheating field are derived are presented. The field of first vortex penetration is measured under DC and RF fields. The combined results confirm that Nb3Sn cavities are indeed operated in a metastable state above the lower critical field but are currently limited to a critical field well below the superheating field.

Point contact tunneling (PCT) and low energy muon spin rotation (LE-µSR) are used to probe, on the same samples, the surface superconducting properties of micrometer thick niobium films deposited onto copper substrates using different sputtering techniques: diode, dc magnetron (dcMS) and HIPIMS. The combined results are compared to radio-frequency tests performances of RF cavities made with the same processes. Degraded surface superconducting properties are found to correlate to lower quality factors and stronger Q slope. In addition, both techniques find evidence for surface paramagnetism on all samples and particularly on Nb films prepared by HIPIMS. I. CURRENT LIMITATIONS OF NIOBIUM ON COPPER CAVITIESSuperconducting cavities prepared by coating a micrometer thick niobium film on a copper substrate enable a lower surface resistance compared to bulk niobium at 4.5 K the operation temperature of several accelerators using this technology, like the LHC or the HIE-Isolde at CERN. Additionally Nb/Cu cavities are more costeffective and do not require magnetic shielding. Thermal stability is enhanced avoiding quenches [1].Despite these advantages, this technology is currently not being considered for accelerators requiring highest accelerating gradient E acc or lowest surface resistance R S at temperatures of 2 K and below. The reason for this is that R S increases strongly with E acc . The origin of this field dependent surface resistance has been the subject of many past and recent studies [2][3][4][5], but is still far from being fully understood. The film thickness of about 1.5 µm is large compared to the London penetration depth λ L of about 32 nm. Differences in the performance (apart from thermal conductivity issues [6]) should therefore be correlated to the manufacturing procedure and the resulting surface structure. Since no single dominant source can be expected, several * Tobias.Junginger@helmholtz-berlin.de † TRIUMF Canada's National Laboratory for Particle and Nuclear Physics, Vancouver hypotheses need to be addressed individually to identify their origin and possibly mitigate their influence on the surface impedance and SRF cavity performances. A. Surface magnetism, a possible source of RF dissipationA possible source of dissipation under RF fields is surface magnetism in the oxide layer; magnetic impurities have a well known deleterious effect on superconductivity, causing inelastic scattering of the Cooper pairs [7] that increase the surface resistance R S and lower the quality factor Q ∝ 1/R S of superconducting RF cavities. The presence of localized magnetic moments on bulk niobium samples was revealed for the first time by Casalbuoni et al. in 2005 [8] by SQUID magnetometry. The AC magnetic susceptibility was measured in an external magnetic field of 0.7 T on Nb samples prepared with standard SRF cavity treatments, i.e. buffered chemical polishing (BCP), electropolishing (EP) and low temperature baking at about 120• C. In all cases, the samples displayed a Curie-Weiss behavior below 50 K indicativ...