Experimental investigations on superconducting niobium cavities at highest radiofrequency fields

In order to increase the maximum accelerating gradient in superconducting niobium cavities, electrolytic polishing was applied to 1.3 GHz single cell and nine-cell cavities. A facility for the electropolishing was set up in collaboration with CERN. Tests on single-cell cavities have shown that electropolishing increases the maximum accelerating gradient significantly as compared to the standard etching treatment. Preliminary results on high temperature heat treatments of electropolished cavities are shown. Electropolishing of nine-cell cavities was carried out in collaboration with KEK. One nine-cell cavity improved the accelerating gradient from 22 MV/m after standard etch to 32 MV/m after electropolishing. A low temperature 'in-situ' bakeout at 120 C for 48 hours was found to be necessary to reduce the degradation of the quality factor at accelerating fields above 25 MV/m. The detailed nature of this process is yet to be understood. Some models were investigated to describe this behaviour. (orig.)