Open Accesn Version

Abstract:
Several studies showed that thermocurrents generate a magnetic field in a horizontal cavity test assembly or cryomodul, which may get trapped during the superconducting (sc) phase transition [1-3]. The trapped flux causes additional dissipation in the order of 1 to10 nΩ during operation and can therefore significantly degrade the quality factor in a TESLA cavity. We simulated the distribution of the generated magnetic field over the whole cavity-tank system for an asymmetric temperature distribution. The asymmetry allows the field to penetrate the RF surface which would be field free in the symmetric case. The calculated results complemented a direct measurement of trapped magnetic flux inside the cavity with a small number of field probes. Finally, the obtained data was combined with RF measurements in three passband modes to determine the overall distribution of trapped magnetic flux due to thermocurrents