Metallurgical analysis and RF losses in superconducting niobium thin film cavities

Bloess, D.; Durand, C.; Mahner, E.; Nakai, Harutaka; Weingarten, W.; Bosland, P.; Loyen, L. van

doi:10.1109/77.620927

Cited by 10 publications

(8 citation statements)

References 4 publications

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“…Two samples of such films were measured inductively at Wuppertal University after dissolution of the copper substrate in a nitric acid solution [7]. The result, 0.45 T < H c2 (4.2 K) < 0.85 T, was much lower than obtained from earlier measurements performed in the presence of the copper substrate [22]. It gave no support to the conjecture that very large values of H c2 (in excess of 10 T) could explain the low values taken by R 0 fl in standard films [6] and clearly demonstrated the difficulty of performing reliable inductive measurements of H c2 in the case of niobium films kept on their substrate.…”

Section: The Upper Critical Fieldmentioning

confidence: 59%

Fluxon pinning in niobium films

Benvenuti

Calatroni

Darriulat

et al. 2001

Physica C: Superconductivity

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Section: The Upper Critical Fieldmentioning

confidence: 59%

Fluxon pinning in niobium films

Benvenuti

Calatroni

Darriulat

et al. 2001

Physica C: Superconductivity

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“…Samples prepared by sputter coating have impurity concentrations about 10-100 times larger than RRR niobium sheets. The total impurity concentration of the samples analyzed here can be estimated to be in the 1000 ppm range [32]. A sample with such a high interstitial impurity concentration has been analyzed in [44].…”

Section: Longitudinal Field Studies Of the Hipims Samplementioning

confidence: 99%

“…The microscopic origin for these degraded properties is unknown yet, there are however major structural and chemical differences between Nb films grown on Cu and bulk Nb samples: the average grain size ≈50-1000 nm is two to three orders of magnitude smaller than for fine grain bulk niobium and the impurity concentration [32] (mostly C and O) is about 10-100 times larger in Nb films. It is therefore possible that the low Δ junctions corresponds to grain boundaries regions with weakened superconductivity due to impurity segregation or porosities near the surface as seen on some thin Nb film cross sections [18].…”

Section: Point Contact Tunneling (Pct)mentioning

confidence: 99%

A low energy muon spin rotation and point contact tunneling study of niobium films prepared for superconducting cavities

Junginger

Calatroni

Sublet

et al. 2017

Supercond. Sci. Technol.

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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...

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“…19 Larger values of of about 70-100 nm have been measured on Nb films of different thicknesses from 100 nm 20 up to a few micrometers. 21 Hsu and Kapitulnik 22 obtained = 160 nm on 2 nm thick single-crystal Nb film. Later, using the two-coil technique, Wang et al 23 demonstrated a similar value ͑166 nm͒ of the penetration depth for 20 nm thick Nb film and smaller Ϸ 95 nm for a thicker ͑90 nm͒ film.…”

Section: Introductionmentioning

confidence: 97%

Dependence of magnetic penetration depth on the thickness of superconducting Nb thin films

et al. 2005

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In this paper we present the results of a systematic study on the magnetic field penetration depth of superconducting niobium thin films. The films of thicknesses ranging from 8 to 300 nm were deposited on a Si substrate by dc magnetron sputtering. The values of the penetration depth ͑0͒ were obtained from the measurements of the effective microwave surface impedance by employing a sapphire resonator technique. Additionally, for the films of thickness smaller than 20 nm, the absolute values of ͑0͒ were determined by a microwave transmission method. We found that the reduction of the film thickness below 50 nm leads to a significant increase of the magnetic field penetration depth from about 80 nm for 300 nm thick film up to 230 nm for a 8 nm thick film. The dependence of the penetration depth on film thickness is described well by taking into account the experimental dependences of the critical temperature and residual resistivity on the thickness of the niobium films. Structural disordering of the films and suppression of superconductivity due to the proximity effect are considered as mechanisms responsible for the increase of the penetration depth in ultrathin films.

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Metallurgical analysis and RF losses in superconducting niobium thin film cavities

Cited by 10 publications

References 4 publications

Fluxon pinning in niobium films

Fluxon pinning in niobium films

A low energy muon spin rotation and point contact tunneling study of niobium films prepared for superconducting cavities

Dependence of magnetic penetration depth on the thickness of superconducting Nb thin films

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