Author: Romanenko, A.S.
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MOPFDV009 On the Nature of Surface Defects Found in 2/0 N-Doped 9-Cell Cavities 336
  • A. Cano, D. Bafia, A. Grassellino, J. Lee, M. Martinello, A.S. Romanenko, T. Spina, Z-H. Sung
    Fermilab, Batavia, Illinois, USA
  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)
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
  • A. Cano, D. Bafia, A. Grassellino, J. Lee, M. Martinello, A.S. Romanenko, T. Spina, Z-H. Sung
    Fermilab, Batavia, Illinois, USA
  • E.A. Karapetrova
    ANL, Lemont, Illinois, USA
  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
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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Cavity Performance Limitations Understanding via SIMS Studies of Cavity Cutouts  
  • A.S. Romanenko
    Fermilab, Batavia, Illinois, USA
  The performance limitations of the modern SRF cavities are related to the near-surface cavity material structure, including the distribution of interstitial impurities such as hydrogen, oxygen, and nitrogen. The most direct way to connect the observed quality factors and achievable gradients to the material structure is to analyze the cavity cutouts using the most suitable analytical instruments and technique. In this contribution I will present the most recent insight gained from such studies using the state-of-the-art time of flight secondary ion mass spectroscopy (TOF-SIMS) on cutouts from the highest gradient-performing cavities, as well as the cavities prepared with the "mid-T" bake. The origins of the observed cavity behavior will be discussed.  
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THPTEV016 The Role of Oxygen Concentration in Enabling High Gradients in Niobium SRF Cavities 871
  • D. Bafia, A. Grassellino, A.S. Romanenko
    Fermilab, Batavia, Illinois, USA
  We studied the role of O concentration with depth in the performance of Nb SRF cavities. An ensemble of electropolished 1.3 GHz cavities, which initially showed high field Q-slope (HFQS), was subjected to sequential testing and treatment with in-situ low temperature baking at various temperatures. We find that increasing the bake duration causes (i) an increase in the onset of HFQS until it is absent up to quench (ii) a non-monotonic relationship with the quench field (iii) an evolution of the RBCS toward a non-equilibrium behavior that drives anti-Q slope. Our data is qualitatively explained by assuming an O diffusion model and suggests that the mitigation of HFQS that arises from 120°C in-situ LTB is mediated by the diffusion of O from the native oxide which prevents the precipitation of proximity-coupled Nb nano-hydrides, in turn enabling higher quench fields. The decrease in quench field for cavities in which O has been diffused >90 nm from the RF surface may be due to a reduction of the field limit in the SS bilayer structure. We also suggest that the evolution of the RBCS occurs due to the absence of proximity coupled inclusions, bringing about non-equilibrium effects.  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2021-THPTEV016  
About • Received ※ 22 June 2021 — Revised ※ 13 September 2021 — Accepted ※ 13 October 2021 — Issue date ※ 23 November 2021
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FROFDV03 Investigating the Anomalous Frequency Variations Near Tc of Nb SRF Cavities 885
  • D. Bafia, M. Checchin, A. Grassellino, A.S. Romanenko
    Fermilab, Batavia, Illinois, USA
  • J. Zasadzinski
    IIT, Chicago, Illinois, USA
  We report recent studies on the anomalous frequency variations of 1.3 GHz Nb SRF cavities near the transition temperature Tc and use them to investigate the underlying physics of state-of-the-art surface treatments. One such feature, a dip in frequency, correlates directly with the quality factor at 16 MV/m and the anti-Q slope that arise in cavities with dilute concentrations of N interstitial in the RF layer achieved via N-doping and mid temperature baking. For N interstitial, we find that the dip magnitude and Tc follow exponential relationships with the electronic mean free path. We present the first observation of the frequency dip near Tc in a cavity baked at 200 C in-situ for 11 hours, which is concurrent with the anti-Q slope, and may be driven by oxygen diffused from the native oxide, thus suggesting the possibility of ‘‘O-doping.’’ We also investigate the conductivities of two cavities that display different resonant frequency behaviors near Tc and suggest that the anti-Q slope and frequency dip phenomena may occur in the presence of interstitial N or possibly O that inhibit the formation of proximity coupled Nb nano-hydrides.  
slides icon Slides FROFDV03 [0.835 MB]  
DOI • reference for this paper ※ doi:10.18429/JACoW-SRF2021-FROFDV03  
About • Received ※ 25 June 2021 — Revised ※ 13 September 2021 — Accepted ※ 18 December 2021 — Issue date ※ 28 April 2022
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9MeV Electron Irradiation on Nb Samples and 1.3GHz SRF Cavity  
  • T. Spina, A. Grassellino, A.S. Romanenko
    Fermilab, Batavia, Illinois, USA
  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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