Prototyping of a multicell superconducting cavity for acceleration of medium-velocity beams

Three 6-cell 805 MHz superconducting cavity prototypes for acceleration in the velocity range of about 0.4 to 0.53 times the speed of light have been fabricated and tested. The quality factors (Q0) were between 7 exp9 and 1.4 exp10 at the design field (accelerating gradient of 8–10 MV/m). The maximum gradients reached were between 11 and 16 MV/m; in each case, the Q0 values were > 3 exp9 at the maximum gradient. The design, fabrication, surface preparation, and rf testing of the 6-cell cavities are reported in this paper.