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.

An overview of air-bearing spacecraft simulators is provided. Air bearings have been used for satellite attitude determination and control hardware veri cation and software development for nearly 45 years. It is interesting to consider the history of this technology: how early systems were rst devised and what diverse capabilities current systems provide. First a survey is given of planar systems that give a payload freedom to translate and spin. Then several classes of rotational air bearings are discussed: those which simulate three-axis satellite attitude dynamics. The subsequent section discusses perhaps the most interesting facilities: those that provide both translational and three-dimensional rotational freedom. The diverse capabilities each style of air-bearing testbed provides, the many settings they can be found in, and ways to improve facility performance are described.

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An overview of air-bearing spacecraft simulators is provided. Air bearings have been used for satellite attitude determination and control hardware veri cation and software development for nearly 45 years. It is interesting to consider the history of this technology: how early systems were rst devised and what diverse capabilities current systems provide. First a survey is given of planar systems that give a payload freedom to translate and spin. Then several classes of rotational air bearings are discussed: those which simulate three-axis satellite attitude dynamics. The subsequent section discusses perhaps the most interesting facilities: those that provide both translational and three-dimensional rotational freedom. The diverse capabilities each style of air-bearing testbed provides, the many settings they can be found in, and ways to improve facility performance are described.

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A b str actNon-contacting interactions between pennanent magnets and superconductors known as "flux pinning" provide a novel way to fix many modular space systems in desired relative positions and orientations. from space stations to close-proximity fonnations. When cooled appropriately, these flux-pin ned interfaces require no power or active control and very lillie mass but provide very high mechanical stiffness (>200 N/m for a few hundred grams of material) and damping (2% of critical) between modules, making the technology ideal for in-orbit assembly applications. We describe new measurements and simulations 10 characterize these values for spacecraft applications. Flux-pinned interfaces have so far achieved inter-module separations in the 8-10 em range with -100 g of mass on each module, with the prospect of larger separations. We also discuss several means to actuate the noncontacting couplers, which is a first step toward the development of devices for the noncontacting manipulation and reconfiguration of modular space systems.

Potential contributions to the residual surface resistance of niobium films exposed to 1.5 GHz microwaves are reviewed and studied. These include the oxidation of the film surface, the formation of hydride precipitates, the contamination by noble gas atoms and the presence of macroscopic film defects such as those resulting from the roughness of the substrate. Particular attention is given to the dependence of the residual resistance on the amplitude of the microwave. Results similar to those obtained for bulk niobium provide strong evidence against the conjecture that the small size of the film grains should be a fundamental limitation to the production of films having a low residual resistance.

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