Critical fields and loss mechanisms in superconducting cavity resonators made of niobium

In 3 GHz single-cell cavities accelerating fields above 20 MV/m are obtained routinely with a maximum of Eacc=31 MV/m. An optimized chemical treatment as well as the improved single-sided titanisation (SST) results in surface fields above 60 MV/m without any field emission. Typically, the quality factor at 1.4 K varies between 1.1010 and 4.1010. The best Q-value of 7.1010 corresponds to a surface reistance of only 4.1 nΩ. Taking into account the BCS-losses and the surface resistance due to frozen-in-flux no residual resistance was present in this test. The nine-cell structures achieved gradients between 8.5 MV/m and 16 MV/m at low field Q0-values above 5.109. The application of a high resolution X-ray diagnostic system showed an accumulation of the emission sites on the lower iris of the cells. An improved assembly procedure resulted on Q0=3.5.1010 and Eacc=17.3 MV/m with a low field emission loading in a final test. Avoiding even traces of hydrocarbon contaminations in the cavity vacuum is crucial for a low residual resistance as well as a high onset of field emission. Pure and dustfree adsorbed gases do not cause a degradation of the RF properties. The quantitative analysis of thermometry data is used for systematic investigations of the local distribution of the residual resistance. (orig./MM)