Studies of RF-superconductivity properties of niobium film-coated cavities at CERN

Extensive tests have been performed on several cavities, operated at 1.5 GHz in the TM010 mode, obtained by sputter-coating niobium on copper, in order to study the physical phenomena responsible for their ultimate performance. Cavities manufactured with the same technique, but resonating at 352 MHz, are in current operation in the CERN Large Electron-Positron Collider LEP. The measured RF data have been analyzed to extract information about: 1) the BCS resistance properties of each film, 2) the residual resistance of sputtered niobium deposited onto copper cavities manufactured with various techniques, and 3) the influence of trapped magnetic fluxons on the film's superconducting RF properties. Several tests have been made also on bulk niobium cavities at 1.5 GHz manufactured through different methods in order to compare and extend the results of the film studies. Analyses confirm that the sputtered niobium films provide: 1) lower BCS resistance than the bulk, 2) lower sensitivity of the surface resistance to external applied magnetic field, 3) residual resistance comparable to the bulk, once the substrate's properties are well controlled.