On breakdown fields of a superconducting niobium cavity at S band

The influence of several surface preparation techniques on both Q0 values and peak rf fields has been studied in a welded beam hole cavity between 2 and 4 GHz. In the TE011 mode peak rf fields up to 810 G at Q0≈1011 have been obtained, whereas, in modes subject to electron loading, the limitations are below 400 G, corresponding to E≲30 MV/m. The significant difference in the breakdown field of modes with and without electron loading show that the observed frequency dependence of Hcrit can easily be explained by electron loading and radiation damage.