An experimental apparatus for accurate characterization of rf properties of superconducting samples near the transition temperature has been designed and realized. It is composed of a standard liquid helium cryostat equipped with a variable temperature insert. A feedback system allows temperature stabilization within 10−2 K around a chosen temperature. A simple, while exhaustive, thermodynamical model of the cryostat operation is presented together with the description of the temperature control system. The rf measurement technique is also presented with the first results obtained on a niobium sample used for calibration.

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