Collaborative project: 05H2015 - R&D ACCELERATORS (SRF). Superconducting radio-frequency cavity developments for future accelerators. Investigation of N-doping and electron field emission to increase the quality of Rb resonators. Final report

Today, superconducting cavity resonators made of high-purity niobium are used in almost all modern accelerators. In order to achieve the acceleration field strengths of about E(acc) - 30 MV/m required in current accelerators as well as a sufficient high field quality Q(0), the surfaces of these cavities must have a carefully worked out combination of mechanical and chemical polishes. Electropolishing and heat treatment are necessary to suppress parasitic electron field emission, which mainly occurs at microscopic roughness or defects. Since planned instruments for high-energy physics like the ILC provide for much higher field strengths in the range of up to 50 MV/m for the cavities, a further improvement of the quality is essential for energy-efficient operation, and also the field emission increasingly occurring with increasing electric field must be further effectively suppressed, so that the optimization of the accelerating structures plays a key role. Within the framework of this project, different strategies for a possible reduction of power dissipation should be investigated. For example, attempts were made to suppress the parasitic field emission of niobium resonator materials by various conditioning processes, especially by targeted heat treatments and ion bombardment. Furthermore, the Nb surfaces were briefly melted by irradiation with short, intensive laser pulses in the UV range in order to smooth defect structures. On the other hand, niobium material samples should be doped with nitrogen by heat treatment. The aim here was to follow the structural changes associated with N-doping in-situ and to establish a correlation to the superconducting properties with the aim of a better understanding of these processes and optimized process control to increase the quality of the cavities. For the systematic investigation of N-doping, a new ultra-high vacuum process chamber has been set up, which allows investigations with X-ray methods in-situ, i.e. during processing. The experiments carried out with this chamber show that the doping can be monitored in a sensitive time-resolved manner, and there were indications that nitrogen is mainly stored on octahedron gaps of the bcc lattice. At the same time, it is also shown that increased process temperatures, as required for N doping, are accompanied by morphological changes of the Nb samples, which cause an increasing parasitic field emission, so that further optimization steps are necessary in order to achieve the operating parameters required for operation in the ILC.