Recent investigations have shown that the superconducting properties of Nb-SRF-cavities can substantially be improved by the exposure of the cavities to a low-pressure nitrogen atmosphere at temperatures between 800 and 1000°C. Such a thermal treatment may lead to an enhancement of the quality factor of the cavity by up to 350%. Up to now, however, the processes evolving during the heat treatments are not at all understood. Especially the atomic and electronic structure of the heat-treated Nb-N has not been investigated in detail, so that the relation to the superconducting RF-properties remains unclear. In the framework of the present project, in-situ investigations of the physico-chemical processes that take place during the doping of Nb with nitrogen using spectroscopic X-ray methods (Extended X-ray absorption fine structure and X-ray absorption near edge structure, EXAFS/XANES) are planned, i.e. the determination of the atomic structures in the cavity materials are obtained during the heat treatments in controlled gaseous atmospheres. For this purpose, a small ultrahigh vacuum chamber should be realized, in which the samples can be exposed to a nitrogen atmosphere at elevated temperatures. The proposed experiments should directly allow the determination of the depth that the adsorbed gas has reached, and identify the resulting structural modifications. Material samples prepared by the DESY facilities and the project partners of Hamburg and Siegen Universities will be investigated on a complimentary basis – those experiments enable comparability as well as reproducibility of the obtained results. In a similar manner, the material samples prepared in this work should be characterized in terms of their superconducting properties by the project partners in Berlin and Hamburg. The final goal is to identify the atomic and electronic structures that improve the quality factor of the cavities.

Duration:
01.07.2015 - 30.09.2018

Project-leader:
Prof. Dr. D. Lützenkirchen-Hecht

Budget:
316.000 €