Calibration of the scanning thermometer resistor system for a superconducting accelerating cavity

A calibration measurement for the temperature mapping system in use at CERN for superconducting accelerating cavities immersed in subcooled liquid helium is presented. It allows a study of cavity losses in a quantitative way by relating the temperature increase (ΔT) at the cavity outside wall to the power dissipated at the inside. The calibration set up simulates an isolated lossy region in a superconducting cavity. The influence of the bath temperature and the orientation of the heated surface have been investigated. ΔT is determined by the heat current density Q at the resistor location according to Δ T = α Q^3/4. The proportionality constant α depends on the particular experimental conditions. The experimental results are compared with heat transfer calculations in subcooled helium. As an application typical loss mechanisms in superconducting cavities are described.