INFN/LNL, Legnaro, Padova
Univ. degli Studi di Padova, Padova
IUAC, New Delhi
The common limitation of systems conceived for the RF characterization of samples consists in the difficulty of scaling the measured results to the real resonator. The revolutionary idea consists in the production of small resonators (6GHz) completely equal in shape to the real scale model. Performing RF tests on a large amount of cavities it is possible to study alternative thin film superconducting materials, traditional and innovative surface treatments. In this framework Nb3Sn films are deposited on the internal surface of 6GHz cavities through the multilayer sputtering method. A UHV magnetron sputtering technique in a post magnetron configuration has been used. A Nb-Sn cathode is maintained in a fixed position, while the 6GHz resonator can be moved up and down thanks to a linear feedtrough. The external coil is mounted in the cathode zone. The cavity is then annealed at 960°C for a few hours (UHV) to obtain the stoichiometric A15 phase. The surface resistance is evaluated through the cavity quality factor measurement at 4.2 K.
INFN/LNL, Legnaro, Padova
Univ. degli Studi di Padova, Padova
IUAC, New Delhi
Nb3Sn has been produced using the liquid tin diffusion method. A bulk Nb 6 GHz cavity is introduced into molten Sn (dipping step) and heat treated (annealing step). The process temperature must be higher than 930°C, to avoid the formation of spurious low Tc phases. The experimental procedure has been progressively modified to obtain a homogeneous, stoichometric and compact film with satisfactory superconducting properties. The "hybrid" process is particularly promising: the sample annealing is partly performed in Sn vapour, partly in vacuum (Tc = 16-17 K and deltaTc = 0,3-0.5 K, no residual Sn traces on the sample surface, no Sn rich phases). A hundred of small 6 GHz cavities, completely equal in shape to the real scale model, were built. Having good results with A15 samples, doesn't mean obtaining performant Nb3Sn superconducting resonators. Several Nb3Sn 6 GHz cavities have been produced and tested with encouraging results.
INFN/LNL, Legnaro, Padova
Univ. degli Studi di Padova, Padova
Among possible Surface Treatments, Electropolishing (EP) occupies a key role, because is the cleanest way for removing hundreds of microns of material. The standard recipe for Niobium EP foresees the utilization of a mixture of H2SO4 and HF. Literature results are excellent, however the EP of thousands of cavities could become an industrial nightmare from the point of view of security at work. HF is not like other highly corrosive acids: if it gets in contact with skin, pain is not felt, but F- ions begin to pass through, searching for the bone calcium. Up to little time ago, it was common opinion that Niobium EP without HF was impossible, unless of explosive mixtures based on perchloric acid. This is not true anymore! Since a few years a green chemistry based on ionic liquids has come to the fore, and our group was the first to EP Niobium by a harmless mixture of Choline Chloride and Urea heated around 150°C. If compared to the HF based recipe, ionic liquids provide higher etching rate and lower surface roughness. In this talk, the application to 6GHz cavity EP will be reported and the addition of a secret third "magic" component to Choline Chloride and Urea will also be revealed.