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For superconducting alloys and compounds, at a given operating temperature, the best rf performances (low surface resistance and high relevant critical fields) are obtained for high Tc and low resistivity materials. Among the possible candidates, A15 compounds appear to be the most promising.
We needed a fast, easy and performing way to characterize A15 superconducting materials for their potential application to accelerating resonators. The idea is to build microcavities completely equal in shape to the real scale model. The rf characterization of samples is an useful diagnostic tool to accurately investigate local properties of superconducting materials. However, a common limitation of systems used for this, often consists in the diculty of scaling the measured results to the real resonator.
In this work we will proof that 6 GHz resonators can simply become our cavity shaped samples. Our attention was focused on two materials: V3Si that has a really high RRR value and Nb3Sn that is the only A15 material already used for a resonant accelerating structure.
The process parameters optimization necessary to improve the A15 phase superconducting properties, crystal structure and morphology is going on through the small sample production: this is fundamental but still not enough.
We are perfectly aware that having satisfactory results with A15 samples, doesn't mean obtaining good superconducting cavities with ease. Our solution is to work directly with cavities. Obviously using 1.5 GHz resonant structures would be time wasting and a cost limited approach. In the best situation, working very hard, one can produce and measure one resonator every two weeks.
6 GHz cavities are made from larger cavities fabrication remaining material, they don't need welding (even for anges) and they can be directly measured inside a liquid helium dewar. Finally it is possible to perform more than one rf test per day!