Nb thick films for superconducting radiofrequency cavities

One of the main challenges in the construction of new particles accelerators and the upgrading of the existing ones, is the cost reduction. The development of the copper cavities coated with niobium has been under investigation for the last 30 years as the most important alternative to the Nb bulk cavities. The Nb on Cu cavities technology has been applied in different accelerators (ALPI at LNL; ISOLDE, LHC at CERN), although the strong characteristic Q-slope which limits their use to low accelerating gradients applications. Recently, it has been proven the possibility to mitigate the Q-slope, characteristic of the Nb on Cu cavities, in prototype 6 GHz cavities. The methodology used to reach constant high RF performances at low accelerating gradients consists in a Nb thick film (~45 μm or higher) on a Cu cavity by DC magnetron sputtering technique at high temperature (550° C). This method has shown systematic improvements in the Q factor and in accelerating gradients when it is applied in a long pulse manner, and it has shown to promote a bulk-like microstructure. During this work, different improvements on systems and procedures have been applied to improve thick films performances and characterizations: vibrotumbling technique to improve the cavities’ surface, a coating system simplification, and a measurement system installation for the trapped magnetic flux study have been implemented. In addition, thick films have been studied in different configurations: 6 GHz elliptical cavities, planar samples and a QPR sample. The thick films on cavities demonstrated to be a dense, void-free, Nb bulk-like morphology (showed by EBSD and SEM) with good superconducting behavior (showed in DC magnetometry). The thick film deposited on a Cu QPR sample, using the same approach of the 6 GHz cavities (high temperature, long pulse DC magnetron sputtering), was characterized at different frequencies, and showed unprecedent surface resistance values for a coated sample, confirming the capability of thick films to mitigate Q-slope in Nb-Cu SRF cavities.