Electropolishing and in-situ Baking of 1.3 GHz Niobium Cavities

Three electropolished one-cell cavities were measured before and after in-situ bakeout under ultra-high vacuum conditions. Before bakeout the cavities showed a strong reduction in quality factor at fields above 25 MV/m. After the bakeout the Q drop was no longer present and gradients of up to 39 MV/m were achieved. This indicates that electropolishing yields highest accelerating gradients in niobium cavities only in combination with in-situ bakeout.