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CORNELL LABORATORY FOR ACCELERATOR-BASED SCIENCES AND EDUCATION

Study of the High Field Q-Slope Using Thermometry

Grigory V. Eremeev, Ph.D.

Cornell University 2008

Abstract

The high Field Q-slope and the mild baking effects in 1.5 GHz superconducting radio frequency niobium cavities were studied experimentally within the framework of current models, which are the interface tunnel exchange, the magnetic field enhancement and the modified oxygen pollution models.

Regarding the high field Q-slope, we found that the high field Q-slope in 1.5 GHz superconducting niobium cavities starts at 105±5 mT in all cavities after chemical treatment. The thermometry system indicated that the high field Q-slope is a magnetic field effect. It also showed that it is not a local effect, i.e. all thermometers in the magnetic field region show non-quadratic losses at fields above 105±5 mT, the high field Q-slope. The slope in a given region depends on the strength of magnetic field, i.e. it is bigger, where the magnetic field is stronger.

Regarding the mild baking effect, we dicovered thet the mild baking effect in the case of 100°C baking for 48 hours occurred to the depth of about 20 nm for cavities treated by EP and by BCP.

Regarding the present models for studied effects, we conclude that neither of them give a satisfactory explanation of the data collected: the interface tunnel exchange failed to account for the preferential heating un the magnetic field regions, the magnetic field enhancement model failed to account for high-field-Q-slope's heating in areas without grain boundaries, the modified oxygen pollution model failed to account for the high temperature baking. As an alternative we propose a new model for the mild baking effect, an oxide-purifier model.


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