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High Field Q-slope of a Nearly Oxide-Free Niobium Cavity

High Field Q-slope of a Nearly Oxide-Free Niobium Cavity. Correlation between XPS and temperature maps. Grigory Eremeev Cornell University Cornell Laboratory for Accelerator-based Sciences and Education . Outline. Introduction XPS results Cavity results Conclusion. Oxygen pollution model.

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High Field Q-slope of a Nearly Oxide-Free Niobium Cavity

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  1. High Field Q-slope of a Nearly Oxide-FreeNiobium Cavity Correlation between XPS and temperature maps Grigory Eremeev Cornell University Cornell Laboratory for Accelerator-based Sciences and Education

  2. Outline • Introduction • XPS results • Cavity results • Conclusion Grigory Eremeev

  3. Oxygen pollution model H. Safa, K. Kowalski, G. Ciovati Grigory Eremeev

  4. Previous studies IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-23, NO. 2, MARCH 1987 We will get rid of the oxide by a high temperature baking, F. Palmer did it at 14000C, recent XPS data show 4000C removes oxide Grigory Eremeev

  5. Destruction of pentoxide • 4000C cause dissolution of oxide within half an hour • After that less than a nm of oxide is on the surface Residual Oxygen on Nb Heated to 500 C R.E. Kirby, F.K. King, and H. Padamsee, SLAC-TN-05-049, May 2005 Grigory Eremeev

  6. No oxidation in vacuum ~ 12 hours Sample was left in vacuum overnight, yet no reoxidation in vacuum! Residual Oxygen on Nb Heated to 500 C R.E. Kirby, F.K. King, and H. Padamsee, SLAC-TN-05-049, May 2005 Grigory Eremeev

  7. Surface composition • After 4000C baking all Nb2O5 is gone • Main constituents on the surface are Nb and NbO Residual Oxygen on Nb Heated to 500 C R.E. Kirby, F.K. King, and H. Padamsee, SLAC-TN-05-049, May 2005 Grigory Eremeev

  8. Oxygen contamination • Improved oxygen diffusion model to explain the effect of low-temperature baking on high field losses in niobium superconducting cavities, Gianluigi Ciovati, Applied Physics Letters 89,022507 (2006) Generation term Pollution layer Oxygen concentration in London penetration depth(~ 40 nm) after 4000C/2 hours baking is less than it is after 1450C/45 hours baking Grigory Eremeev

  9. Baking setup Water cooled copper clamps to keep temperature low on the flanges Steel box filled with argon under pressure Two 1.5 kW band heaters Grigory Eremeev

  10. Baking temperature profile T > 400oC for ~ 2 hrs Temperature on the flange Grigory Eremeev

  11. First results! • Surface resistance increases - 8.9/10.7 nOhm • Medium field Q-slope(γ) decreases 2.18/1.80 • High field Q-slope is present LE1-36, ~ 1-3 mm grain size, VEP + flash BCP Grigory Eremeev

  12. Temperature maps are similar Before baking at 400oC After baking at 400oC Grigory Eremeev

  13. Anomalous high-loss regions Three new high-loss regions(< 10 % of the surface) had high residual resistance, ~ 100 nOhm Grigory Eremeev

  14. High field Q-slope All other regions after 4000C baking show the high field behavior similar to that before baking Grigory Eremeev

  15. Contradiction to oxygen model The calculations showed that oxygen content is low in the penetration depth after high temperature heat treatment, but quality factor and the thermometry measurements showed, that the high field Q-slope is still present after the treatment. So oxygen diffusion explanation for high field Q-slope improvement is contradicted. Grigory Eremeev

  16. Saclay experiment There are other evidences that oxygen pollution layer is not responsible for high field Q-slope • Saclay, EP cavity + Bake (2004), No high-field Q-slope • Remove oxide by HF and grow new oxide • High-Field Q-slope does not return with new oxide (and presumably new interface – pollution layer) Grigory Eremeev

  17. Shorter baking time, less benefit But there are also evidences that diffusion does play a role Grigory Eremeev

  18. Diffusion rates Could there be another impurity that diffuses? from “Niobium: physico-chemical properties of its compounds and alloys”, Atomic Energy Review, Vienna, 1968. Grigory Eremeev

  19. Another mechanism The bulk diffusion rates may not be applicable for diffusion near the surface! Nb Oxidation Nb2O5 Nb 1000C baking Nb2O5 Nb Nb 4000C baking Grigory Eremeev

  20. Conclusion • High field Q-slope does not change after 4000C baking, but calculations show, that the oxygen concentration in penetration depth should be small, so oxygen diffusion explanation for high field Q-slope improvement is contradicted • Remaining sub-oxides after 4000C baking did not increase the residual resistance(10 nOhm) • Other diffusion mechanisms may be involved in the baking effect. • XPS, SIMS measurements are necessary, which will not concentrate only on oxygen Grigory Eremeev

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