High field q slope of a nearly oxide free niobium cavity
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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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High field q slope of a nearly oxide free niobium cavity

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


Outline

Outline

  • Introduction

  • XPS results

  • Cavity results

  • Conclusion

Grigory Eremeev


Oxygen pollution model

Oxygen pollution model

H. Safa, K. Kowalski, G. Ciovati

Grigory Eremeev


Previous studies

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


Destruction of pentoxide

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


No oxidation in vacuum

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


Surface composition

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


Oxygen contamination

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


Baking setup

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


Baking temperature profile

Baking temperature profile

T > 400oC for ~ 2 hrs

Temperature on the flange

Grigory Eremeev


First results

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


Temperature maps are similar

Temperature maps are similar

Before baking at 400oC

After baking at 400oC

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Anomalous high loss regions

Anomalous high-loss regions

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

Grigory Eremeev


High field q slope

High field Q-slope

All other regions after 4000C baking show the high field behavior similar to that before baking

Grigory Eremeev


Contradiction to oxygen model

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


Saclay experiment

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


Shorter baking time less benefit

Shorter baking time, less benefit

But there are also evidences that diffusion does play a role

Grigory Eremeev


Diffusion rates

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


Another mechanism

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


Conclusion

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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