Nb thick films in 6 GHz superconducting resonant cavities – Cristian Pira PhD Thesis

Author: Cristian Pira; Type of thesis: PhD Thesis
Abstract: A key challenge for the next accelerators is the cost reduction. Bulk niobium cavities performances are closer to their theoretical limits and an alternative technology is mandatory. Niobium thin film copper cavities are the most explored solution, but the Q-slope characteristic of these resonators limits the applications where high accelerating fields are requested. In this work an original approach is adopted in order to enhance the performances of sputtered cavities, exploring the possibility to sputter 70 micron thick films to straighten up the Q-slope in Niobium sputtered copper cavities. The engineering of Nb thick films deposition on 6 GHz copper cavities, necessary to minimize the stress on niobium thick films and avoid film peeling from the substrate, is reported. Thick films show RRR values over 60. Different strategies have been used for stress reduction: deposition at high temperature of 550 °C, deposition at the zero stress pressure point (5∙10-2 mbar), and the development of a multilayer deposition procedure. Rf tests at 4,2 K and 1,8 K on 6 GHz resonant cavities are the tool used to evaluate the influence of the critical deposition parameters on the cavity performances. Two different venting procedures have been tested: a standard one, and a high temperature one, in which pure nitrogen is inserted in the vacuum chamber immediately after the end of the sputtering process. Thick films open the possibility to post treatments on sputtered cavities: the effect of buffer chemical polishing and heat treatments at different temperature have been investigated. The Rf tests on 6 GHz cavities show the fundamental role played by the surface preparation and the possibility to enhance the maximum accelerating field minimizing the film stress. Above all, for the first time, the possibility to obtain flat curve of Q-factor versus accelerating field in sputtered cavities with thick films it is demonstrated.