Investigations of a Superconducting Beam Tube Niobium Cavity at S-Band

The influence of several surface preparation techniques like electropolishing, chemical edging, anodizing and UIIV-heat treatment on both the Q-value and the peak RF fields have been studied in a welded S-band cavity having large beam tubes. Peak magnetic fields up to 650 Oe at a Q-value of 8×1010 in the TE011-mode and peak surface electric fields of 25 MV/m at Q = 7×109 in the TM010-mode have been obtained. Simultaneous measurements of the mean free path in the surface layer of the cavity permit to calculate Hc1. The results indicate that Hc1 is no limitation for the magnetic fields achievable in RF-cavities. This makes type-II-superconductors like Nb3Sn interesting for practical applications at high frequencies.