Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Elektronen-Speicherring BESSY II, Berlin
For CW applications of superconducting cavities, obtaining a high quality factor is an important issue, since the required cryogenic power drops inversely proportional to Q0. Q0 is limited by BCS-losses and static losses, i.e. impurities and trapped magnetic flux. With sufficient magnetic shielding for TESLA type cavities, typical values of 2*1010 are being achieved at 1.8K operating temperature. The flux trapping was monitored by cooling the cavity at different ambient magnetic fields. In another experiment we have observed a significant increase in the Q0 value of up to 50% when subjecting the cavity to an additional cryogenic cooling cycle. Quantitative results are being presented and possible explanations for the effects are being discussed.
