SURFACE SCANNING THERMOMETERS FOR DIAGNOSTIC PURPOSE OF THE TESLA SRF CAVITIES

specially developed supecfluid helium cooled surface scanning thermometers have been constructed and successfully used as diagnostic probes of anomalous RF losses lIXl Thermal' Breakdown on lESLA SRF cavities. We report about these thermometers calibration (i.e Thermal response (Ll1) vs the Heater Power (Q» in supecfluid helium. Two different test-cells were used for this purpose: 1) the cell#1 wbich is equipped with a Joule-heated Niobium plate lIXl designed to support a set of up to 16 thermometers simultanousely, 2) the cell #2 consisting of a Joule-heated OFHC copper rod , equipped with 3 calibrated carbon resistors in the temperature range 1.5 K-20 K and placed in a vacuum can, which is used for precise calibration of one surface scanning thermometer in a large heat flux q range ( i.e 10 mW/cm 2 :S;q:S; 3 W/cm 2 ). The ftrst test-cell allows us to get statistical information about the Ll T vs Q curves for different thermometers. Moreover, by comparison of the mean value of the surface scanning thermometers calibration curve (i.e Ll T vs Q) with the numerical simulation results orland with a reference thermometer thermal response, an estimation of the thermometer measurement efficiency 1\ is obtained for different mounting conditions (i.e with vs without the use of a thermal bonding agent between the thermometer tip and the test-specimen cold surface) of the temperature sensors on the test-specimen. Also are discussed the effect of the supecfluid belium (He m bath temperature Tbath on the thermometer measurement efficiency 1\. The calibration results obtained with the cell #2 show strong non-linearities in the Ll T vs q curves for the scanning thermometers when used without a thermal bonding agent : the thermometer measurement efficiency 1\ increases strongly with q. The temperature maps obtained on lESLA cavities are successfully analysed using the present calibration data and numerical simulations results peIformed with different codes including a Finite Element Thermal Code.