Deposition and Characterisation of Niobium Films for SRF Cavity Application

Niobium coated copper cavities are an interesting alternative to bulk niobium ones for Superconducting Radio Frequency (SRF) applications to particle accelerators. The magnetron sputtering is the technology developed at CERN for depositing niobium films and applied over the past twenty years. Unfortunately, the observed degradation of the quality factor with increasing cavity voltage, not completely understood, prevents the use of this technology in future large accelerators designed to work at gradients higher than 30 MV/m, with quality factors of the order of 1010 (or higher). At the beginning of the new millennium some new deposition techniques have been proposed to overcome the difficulties encountered with the sputtering technique. This paper compares the properties of niobium films obtained with the magnetron sputtering and with a cathodic arc deposition in ultra-high vacuum (UHVCA). The UHVCA-produced Nb films have structural and transport properties closer to the bulk ones, providing a promising alternative for niobium coated, high-voltage and high-Q copper RF cavities, with respect to the standard magnetron sputtering technique. Preliminary results and possible approaches to whole cavity UHVCA coating will be presented and discussed.