Improvement of the polishing treatment for niobium surfaces of superconducting cavity resonators

For superconducting cavities made from niobium sheet, which are interesting in the field of accelerator applications, smooth and defect free inner surfaces are needed to achieve a high unloadedQ along with high accelerating field strength. These can be obtained using chemical polishing procedures. Whereas normally a mixture of HF, HNO₃, and H₃PO₄ is applied, we chose the known bath containing HF, HNO₃, and H₂SO₄ and worked above 50 °C. The surface quality was judged by visual inspection using a light microscope as well as by microwave measurements of one spherical resonator. The new method gave less grain boundary etching and, with high field levels, an enlarged unloadedQ.