Surface scanning thermometers for diagnosing the TESLA SRF cavities

In order to investigate the field emission and the thermal breakdown of 9-cell TESLA SRF cavities, 150 specially developed surface scanning thermometers have been built. The description of the thermometers and their calibration in superfluid helium are presented. A special test chamber equipped with a heated niobium plate is used to study the thermometer thermal response versus the heater power at different bath temperatures. The comparison of thermometer response with numerical simulations results and experimental data obtained with reference thermometers mounted on the Nb plate using a thermal bonding agent, allows to get an estimation of the measurement efficiency of scanning thermometers. Experimental data obtained with cavities are analysed with the help of the calibration results and numerical simulations.