Impact of Trapped Flux and Thermal Gradients on the SRF Cavity Quality Factor

The obtained Q0 value of a superconducting niobium cavity is known to depend on various factors like the RRR of the Niobium material, crystallinity, chemical treatment history, the high-pressure rinsing process, or effectiveness of the magnetic shielding. We have observed that spatial thermal gradients over the cavity length during cool-down appear to contribute to a degradation of Q0. Measurements were performed in the Horizontal Bi-Cavity Test Facility (HoBiCaT) at HZB on TESLA type cavities as well as on disc- and rod-shaped niobium samples equipped with thermal, electrical and magnetic diagnostics. Possible explanations for the effect are discussed.