Study of niobium sputtered copper accelerating cavities as possible alternative to the bulk technology-Magistralis Degree Thesis Martina Martinello

Author: Martina Martinello ; Type of thesis: Magistralis Degree Thesis
Abstract: Several studies regarding the radiofrequency (RF) superconducting proprieties of thin film Niobium on Copper cavities are presented. Two types of Niobium on Copper cavities are analyzed: Quarter Wave Resonators (QWRs) and elliptical 6GHz cavities, in both cases a superconducting Niobium film is deposited via sputtering on the Copper substrate.
The QWRs were characterized at CERN, in the framework of a Summer Student Program. Such cavities are nowadays under study at CERN for the project HIE-ISOLDE. The purpose of this project is the upgrading of ISOLDE facility by the implementation of superconducting accelerating cavities in its linear acceleration section. The RF characterization of QWRs is performed in order to evaluate if the resonator reaches the HIE-ISOLDE specification. In addition, for a more complete understanding of the cavity surface properties, the RRR was evaluated by analyzing the resonance frequency shift near the critical temperature.
The 6GHz Niobium on Copper cavities were instead studied at LNL, INFN. The research was carried out in order to obtain a Niobium thin film with good RF superconducting performances, working on the sputtering parameters. The goal was reached with a high temperature Niobium deposition in which the substrate was heated at 400C. This cavity has the highest Q-factor values found for the analyzed 6GHz cavities.
Furthermore, since the Q-slope is the main problem of the Niobium on Copper cavities, which limits the possibility of the implementation in very high gradient accelerators, a new model is proposed as a possible explanation of this issue. This model analyses the Q-slope as a thermal problem and highlights that the lowering of the performances is due to the Niobium/Copper interface. Because of this interface, a pronounced temperature rinsing occurs, which makes worse the superconducting properties.
Moreover, elevated values of temperature were found in the Helium bath near the external cavity surface, during the measurement of the Cu1_3 cavity. Hence, it may be not surrounded by super-fluid Helium but by a vapor Helium layer instead, and the heat exchange may take place, therefore, following the film boiling regime.