Large Grain Superconducting RF Cavities at DESY

The DESY R&D program on cavities fabricated from large grain niobium explores the potential of this material for the production of approx. 1000 nine‐cell cavities for the European XFEL. The program investigates basic material properties, comparing large grain material to standard sheet niobium, as well as fabrication and preparation aspects. Several single‐cell cavities of TESLA shape have been fabricated from large grain niobium. A gradient up to 41 MV/m at Q0 = 1.4⋅1010 (TB = 2K) was measured after electropolishing. The first three large grain nine‐cell cavities worldwide have been produced under contract of DESY with ACCEL Instruments Co. The first tests have shown that all three cavities reach an accelerating gradient up to 30 MV/m after BCP (Buffered Chemical Polishing) treatment, what exceeds the XFEL requirements for RF test in the vertical cryostat.