RESULTS ON LARGE GRAIN NINE-CELL CAVITIES AT DESY: GRADIENTS UP TO 45 MV/M AFTER ELECTROPOLISHING

Since 2009 a series of eight nine-cell cavities (AC151 – AC158) of TESLA shape fabricated of large grain (LG) niobium material [1] is under preparation and test at DESY. In a first step all cavities were tested after a BCP treatment. In a second step additional electropolishing is applied to all cavities. In this paper the treatment will be discussed and present results will be reported. MATERIAL AND FABRICATION Development of LG disc production was done within the framework of the European XFEL driven RD the roughness was small compared to conventional fine grain sheets. Three cavities (AC151 – AC153) have been fabricated from material with RRR of about 420. For five cavities (AC154 – AC158) the RRR of the material was about 350. More details about the material are presented in [3]. For the cavities AC155 and AC156 the steps at the grain boundaries, which appeared after deep-drawing, were smoothened by grinding the RF surface of the halfcells. The complete fabrication was done by Research Instruments GmbH (RI).