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Surface Analysis of Samples Dissected from a Cavity with a High-Field Q-Slope

Surface Analysis of Samples Dissected from a Cavity with a High-Field Q-Slope. Alexander Romanenko Cornell University Cornell Laboratory for Accelerator-based Sciences and Education (CLASSE). Outline.

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Surface Analysis of Samples Dissected from a Cavity with a High-Field Q-Slope

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  1. Surface Analysis of Samples Dissected from a Cavity with a High-Field Q-Slope Alexander Romanenko Cornell University Cornell Laboratory for Accelerator-based Sciences and Education (CLASSE)

  2. Outline • Goal of the study is to improve the understanding of the high field Q-slope in cavities prepared by buffered chemical polishing (BCP) • First attempt to directly correlate surface analysis with T-maps from Q vs. E curves • Cavity results and cutting • Roughness analysis (optical profilometry) • Electron backscattered diffraction (EBSD) results – crystal orientation mapping • XPS and AES (Auger Electron Spectroscopy) results • Conclusions and future experiments Alexander Romanenko

  3. High Field Q Slope Temperature map shown • f = 1.5 GHz • 1 mm grain size • BCP 100 mm Alexander Romanenko

  4. Temperature Mapping dT, mK Hot log(T) “Cold” log(Epk) Epeak = 50 MV/m, Hpeak = 123 mT • Temperature map indicates high field Q-slope in the entire magnetic field region • Some regions (hot) have more pronounced high field Q-slope [Why?] Alexander Romanenko

  5. Contour Plot Dissected 10 hot and 9 “cold” regions C10 C8 H6 H4 C7 H10 C3 C4 H2 C9 H7 C1 C5 C2 H1 H3 H5 H9 H8 Alexander Romanenko

  6. Cutting • Milling machine was used to cut the samples (only water as a lubricant) • Saclay studies showed that high field Q-slope did not change after exposure to air and water • Auger studies on a test sample showed no contamination due to cutting (except for small increase in C) Alexander Romanenko

  7. Techniques • Optical profilometry – roughness comparison • X-ray photoelectron spectroscopy (XPS) – near-surface (a few nm) elemental composition and chemical state • Electron back-scattered diffraction (EBSD) – crystal orientation mapping • Auger electron spectroscopy (AES) • Secondary ion mass spectrometry (SIMS) Alexander Romanenko

  8. Microroughness 850 um 640 um s = 1.8 um s = 1.7 um Cold Hot s = 1.5 um s = 1.6 um • Average roughness – s = 1.5-1.8 mm is the same Alexander Romanenko

  9. Optical Profilometry Hot “Cold” 0.5 mm 1 mm Alexander Romanenko

  10. Profilometry Conclusions • Step height distribution in hot and “cold” regions is similar • Roughness does not appear to be responsible for the hotter regions of the high field Q-slope Alexander Romanenko

  11. EBSD • Electron backscattered diffraction Alexander Romanenko

  12. EBSD Results (100) ± 20° H6 (111) ± 20° (110) ± 20° C7 Alexander Romanenko

  13. EBSD Results XPS analyzed spot size C8 H10 hottest H8 C10 H5 H1 Alexander Romanenko

  14. EBSD Conclusions • No correlation observed between the amplitude of the Q-slope and crystal orientations of individual grains Alexander Romanenko

  15. XPS Results • Four different spots 1 mm2 analyzed on each sample • Three hottest samples out of 10 total revealed nitrogen presence in all 4 spots each • Out of 9 “cold” samples one spot showed nitrogen • Possible N chemical state – NO3 (399 eV BE for free nitrogen, 401 eV for NO3) • Baking cavities helps reducing Q-slope • Baking hottest sample @1100C for 48 hours eliminated nitrogen signal • Al Ka 1486.6 eV X-ray source • Information depth ~7 nm Nitrogen Hot Cold Hot after bake Alexander Romanenko

  16. XPS Nb Hi Resolution • Absolutely identical Nb high resolution spectra for all 19 samples (hot and “cold”) • No role of oxide and oxide/metal interface in the Q-slope Nb2+ Hot Nb0 “Cold” • Al Ka 1486.6 eV X-ray source • Information depth ~7 nm, which includes oxide and interface Alexander Romanenko

  17. AES Results Hot Cold Alexander Romanenko

  18. AES Supports XPS • AES confirms the presence of nitrogen on the hot sample, signal level close to the margin of instrument sensitivity • Four spots analyzed on each sample • Information depth ~1 nm Alexander Romanenko

  19. XPS Conclusions • Oxide and metal/oxide interface do not contribute to the high field Q-slope losses • Nitrogen in the form of NO3 might be responsible for the high field Q-slope for BCP cavities • Baking at 1100C for 48 hours eliminated nitrogen • This is in contradiction with the existing nitrogen diffusion reports for bulk niobium • But nitrogen may move faster down the grain boundaries Alexander Romanenko

  20. Future Work Planned • Variable energy XPS (1.5-6 keV) for non-destructive profiling down to 20 nm deep • XPS on smaller spots for comparison between grain boundaries and grains • Repeat dissection and surface analysis for • Electropolished cavity • Large grain cavity • Single grain cavity Alexander Romanenko

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