Purification technique of 6 GHz Tesla type suoerconducting Nb cavities: A new approach for improving RF performances-Master Thesis RAM KRISHNA THAKUR

Author: RAM KRISHNA THAKUR ; Type of thesis: Master Thesis
Abstract: The lowest possible surface resistivity and higher accelerating field are the paramount considerations, hence are obligatory for accelerating cavities. Since, superconducting materials are used to make radio-frequency cavities for future accelerators. In the case of rf cavities, superconductors are being used in order to minimize the power dissipated and increase the figures of merit of a radio-frequency cavity, such as the quality factor and accelerating gradient.
Hence, these could be achieved by improving surface treatment to the cavity, and processing techniques must be analyzed in order to optimize these figures of merit.
The research work reported in this dissertation mainly carried out on tesla type seamless 6GHz Nb and Cu cavities. We have developed two innovative techniques: firstly, for mechanical polishing of cavities, and secondly for purification of these cavities at atmospheric pressure under cover of 4Helium gas (for protection) and at ultra-high vacuum (UHV) system. These cavities are
fabricated by spinning technology to create seamless cavities.
The main advantages of 6 GHz bulk-Nb cavities are saving cost, materials and time to collect statistics of surface treatments and RF test in a very short time scale. Cavities are RF tested before and after high temperature treatment under atmospheric pressure (under cover of inert gas atmosphere to protect inner and outer surface of cavity) inside transparent quartz tube, and under
UHV conditions. Induction heating method is used to anneal the cavity at temperatures higher than 2000°C and close to the melting point of Nb for less than a minute while few seconds at maximum temperature. Before RF test and UHV annealing, the surface treatment processes like tumbling, chemical, electro-chemical (such as BCP and EP), ultrasonic cleaning and high pressure rinsing (HPR) have been employed. High temperature treatment for few minutes at
atmospheric pressure allow to reduce hydrogen, oxygen and other elemental impurities, which effects on cavity Q-factor degradation, hence recovers rf performances of these cavities. This research work will address these problems and illustrate the importance of surface treatments.