Precise measurements of hot S-parameters of superconducting cavities: Experimental setup and error analysis

Superconducting accelerating cavities used in modern particle accelerators change their intrinsic properties when excited to very high field levels close 10 to the critical field where the superconductivity is affected. In this report we describe a test-bench and data analysis procedure to determine the so-called hot S-parameters from strongly driven cavities and use them to quantify the properties of the cavity at varying field levels. The method is based on analysing reflection coefficient for a large number of configurations in a self-excited loop 15 setup and determining the cavity coupling coefficient κ = Q0/Qext as a function of cavity voltage to high accuracy. Since Qext is determined independently and is a constant, from the information of κ the Q-slope can be determined.