| Paper | Title | Page |
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| MOPFDV009 | On the Nature of Surface Defects Found in 2/0 N-Doped 9-Cell Cavities | 336 |
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In this contribution, we present a systematic study on the microstructure of 1.3 GHz 9-cell TESLA type SRF cavity, processed with 2/0 Nitrogen-doping surface treatment, to explain the premature quench phenomena commonly observed in N-doping treated cavities. The microstructure characterization was carried out using Secondary electron images, advanced metallurgical techniques such as EBSD in parallel with chemical information obtained from spectroscopic techniques. The most remarkable difference is observed in the ends-cavities (1 and 9), which showed roughening of the surface, revealing a series of morphologies associated with Nb cubic phase. The cell-to-cell analysis also showed standard features such as pits with different geometry and distribution, located in grains and grain boundaries. The defects found in this system suggest that the standard electropolishing chemical etching was insufficient to eliminate history defects produced during the manufacture of the cavity, without discarding the role of the impurities, N and O, that could have induced the growth of these morphologies.
H. Padamsee, RF superconductivity (Wiley-VCH Verlag GmbH and Co., KGaA, Weinheim, 2009) |
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| DOI • | reference for this paper ※ doi:10.18429/JACoW-SRF2021-MOPFDV009 | |
| About • | Received ※ 29 June 2021 — Revised ※ 11 March 2022 — Accepted ※ 10 May 2022 — Issue date ※ 11 May 2022 | |
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| MOPFDV010 | Microstructure Changes Observed in the Near-Surface Region of SRF Nb Cavities Cutouts upon Cooling/Heating Cycles Using GI-Synchrotron XRD | 339 |
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We have mapped microstructural changes in the near-surface region of Nb from SRF cavity-cutouts upon thermal cycles in the range from 300 to 30 K using grazing incidence synchrotron X-Ray diffraction (GIXRD). Segregation of secondary phases was observed after the thermal cycle, and their nature has been clarified and discussed in view of previous studies on hydrides formation in SRF bulk Nb cavities. The temperature dependence of the relative population of these formed phases was obtained from GIXRD patterns profile fitting. Both, Nb bulk matrix and the new phases formed after cool-down show specific structural features as thermal contraction/expansion, structural transitions, and Nb lattice variation due to the induced strain by precipitates formation. The information derived from this structural study can explain some phenomena as the dissipation at high accelerating field (i.e. High Field Q Slope, HFQS) in the Nb SRF performance as well as new mechanisms never addressed in previous studies.
A Romanenko, F Barkov, LD Cooley, A Grassellino, Proximity breakdown of hydrides in superconducting niobium cavities, Superconductor Science and Technology, 2013 |
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| DOI • | reference for this paper ※ doi:10.18429/JACoW-SRF2021-MOPFDV010 | |
| About • | Received ※ 28 June 2021 — Revised ※ 12 August 2021 — Accepted ※ 21 August 2021 — Issue date ※ 23 September 2021 | |
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| TUPFDV007 | Surface Impedance of Nb3Sn and YBa2Cu3O7-δ in High Magnetic Fields | 416 |
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Funding: This work has been partially carried out within the framework of the EUROfusion consortium, funding from the Euratom research and training programme 2014-18 and 2019-20 under grant agreement No 633053 New potential rf applications of superconductors emerged with the need to operate in high dc magnetic fields (up to 16 T) where vortex motion dictates the response: the beam screen coating of the Future Circular Collider (FCC) [1] and haloscopes, i.e. rf cavities for the axions detection [2]. However, very few data are available in the required regimes. We present in this work measurements of the surface impedance Z up to 12 T on bulk Nb3Sn and YBCO thin films grown by different techniques. The measurements are performed with a dielectric loaded resonator operating at 15 GHz. We obtained the vortex motion resistivity and extracted the high frequency vortex motion parameters [3]: the depinning frequency, the flux-flow resistivity and the pinning constant, as well as their temperature and field dependences. Substantial differences are highlighted in the high frequency pinning properties of the studied materials, providing useful information on possible improvements in view of applications. A comparison with the results obtained in the microwave frequency range at lower fields (up to 1 T) is given. [1] S. Calatroni, IEEE Trans. Appl. Supercond., vol. 26 p. 3500204, 2016. [2] D. Alesini et al., Phys. Rev. D, vol. 99, p. 101101, 2019. [3] J.I. Gittleman and B. Rosenblum, Phys. Rev. Lett., vol. 16, p.734, 1966. |
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| DOI • | reference for this paper ※ doi:10.18429/JACoW-SRF2021-TUPFDV007 | |
| About • | Received ※ 21 June 2021 — Accepted ※ 21 August 2021 — Issue date ※ 02 January 2022 | |
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FROFDV04 |
9MeV Electron Irradiation on Nb Samples and 1.3GHz SRF Cavity | |
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To enhance bulk Nb RF cavity performances at high accelerating field it is important to prevent precipitates formation. Doping and heat treatments can mitigate such effect and the most accredited theory is based on the presence of proximity-coupled niobium hydrides [*]. Irradiation can induce vacancy-2H complexes [**] and in this study, the effects of 9MeV electrons on Nb samples and cavity up to fluences of 1.8x1021e/m2 are investigated. The size and density changes of micro-hydrides before and after irradiation was measured by cryo-laser confocal microscopy and a new analytic technique based on computer vision has been used. A strong reduction in hydrides size was found after irradiation and the hydrides formation temperature was shifted to lower values suggesting a reduction in both the activation energy barrier and the critical radius. We conclude that electron irradiation can indeed prevent the formation of large hydrides in H-loaded Nb bulk sample. 1.3GHz Nb cavity has also been submitted to electron irradiation treatment in stationary mode and Q vs E curves and T-map measurements before and after irradiations recorded the changes induced by radiation on cavity performance.
[*] A. Romanenko et al., Supercond. Sci. Technol. 26 (2013) 035003 [**]J. Cížek et al. PHYSICAL REVIEW B 79 (2009) 054108 |
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