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 $\text{E≲30\hspace{0.17em}}\mathrm{MV}/\mathrm{m}$ . The significant difference in the breakdown field of modes with and without electron loading show that the observed frequency dependence of H_crit can easily be explained by electron loading and radiation damage.