A systematic study of superconducting properties of niobium films sputtered on the inner wall of radiofrequency cavities is presented. The measured quantities include in particular the response to 1.5 GHz microwaves, the critical temperature, the penetration depth and the magnetic penetration field. In addition to films grown in different gas discharges (Xe, Kr, Ar and Ar/Ne mixtures) and to films grown on substrates prepared under different conditions, the study includes also bulk niobium cavities. The surface resistance is analysed in terms of its dependence on temperature, on RF field and, when relevant, on the density of trapped fluxons. A simple parameterisation is found to give a good fit to the data. Once allowance for the presence of impurities and defects is made by means of a single parameter, the electron mean free path, good agreement with BCS theory is observed. The fluxon-induced losses are studied in detail and their dependence on RF field, on temperature and on the density of trapped fluxons is analysed. The residual resistance is observed to be essentially uncorrelated with the other variables, suggesting that it is dominantly extragranular. In occasions very low residual resistances, in the nΩ range, have been maintained over a broad range of RF field, indicating the absence of significant fundamental limitations specific to the film technology in practical applications such as the production of accelerating cavities for particle accelerators.

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Magnesium diboride (MgB 2 ) has a transition temperature (T c ) of 40 K, i.e., about 4 times as high as that of niobium (Nb). We have been evaluating MgB 2 as a candidate material for radio-frequency (RF) cavities for future particle accelerators. Studies in the last 3 years have shown that it could have about one order of magnitude less RF surface resistance (R s ) than Nb at 4 K. A power dependence test using a 6 GHz TE 011 mode cavity has shown little power dependence up to 12 mT (120 Oe), limited by available power, compared to other high-T c materials such as YBCO. A recent study showed, however, that the power dependence of R s is dependent on the coating method. A film made with on-axis pulsed laser deposition (PLD) has showed rapid increase in R s compared to the film deposited by reactive evaporation method. This paper shows these results as well as future plans.

The SNS linac contains superconducting cavities with β .61 and .81 for the acceleration of Hions. Each of the 6cell cavities, resonant at 805 MHz at 2.1 K, is powered by a 550-kW pulsed klystron via a coaxial coupler. The specifications for the fundamental power coupler (FPC) require that it withstand the full klystron power in full reflection for the duration of the RF pulse (1.3 + 0.15 [decay] ms) at a repetition rate of 60 pps with an average power of 48 kW. The FPC must provide a Q ext of 7.3 x 10 5 and 7.0 x 10 5 for the medium-β and high-β cavities, respectively. A design derived from proven experience at other laboratories (primarily at KEK, for Tristan and KEK-B cavities at 508 MHz) has been adopted for the prototypes of the FPCs. The 50-Ω coaxial line is scaled directly from 508 MHz to the SNS frequency of 805 MHz.

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