Author: Lanza Giulia ; Type of thesis: PhD Thesis
Abstract: Superconducting radiofrequency resonators for particle acceleration have become a standard component for particle accelerators. This work described an in-depth study and development of an alternative to the more frequently used bulk niobium cavities: niobium thin film coated into a copper cavity. The first niobium-coated copper cavity was produced at CERN in the early eighties. The sputter technology was chosen first in the pure diode configuration and subsequently in the magnetron configuration. The latter was adopted for the successful series production of the LEP and LHC cavities. In this work an intensive R&D effort has been undertaken to study the coating technique, to improve it and understand the correlation between the coating system applied and the film morphology, the superconducting properties and the RF film quality. Four different coating configurations for sputtering niobium films into 1.5 GHz copper cavities has been explored. First, the standard technique applied for several years at CERN to coat the LEP cavities has been reproduced. Then, in order to improve the Nb film quality,the application of three main ideas to the sputtering process was investigated: i) making niobium atoms imping perpendicularly to the substrate surface, ii) promoting the effect of plasma bombardment on the growing film, iii) increasing the sputtering rate. Therefore, three different and new sputtering configurations are described: the effect of Nb atoms arriving perpendicularly to the substrate was explored either by using a cathode that follows the cavity shape (Large Area Cavity Shaped Cathode) or by increasing the plasma confinement efficiency by means of a target parallel to the magnetic field lines (Ringed Shaped
Cathode). The removal of adsorbed impurities from the film surface and the increase of the film density were investigated by using a biased third electrode that promoted the positive ion bombardment of the growing film. A mixed Bias-Magnetron system was built using a positively charged metal grid which surrounded the cathode. Different film characteristics were studied and compared, focusing mainly on superconducting and resistive properties. Also morphological and microstructural properties were analyzed in a very valuable collaboration with “Interdepartmental Laboratory of Electron Microscopy” (LIME), University of Rome “Roma Tre”, at the Mechanical and Industrial Engineering Department. Four RF test on different accelerating cavities are reported and commented. In addition a 3-cell bulk niobium 1.3 GHz cavity were prepared and measured in order to compare bulk and thin film results. Even though the work is still in progress all of the partial results to-date have been viii Abstract analyzed and commented, in order to extrapolate every possible information. The final result is a global overview of the sputtering coating techniques and a of the results obtained using each system. Suggestions for future efforts have been included as part of the conclusions.