Superconducting Structures for a 130 MeV Recyclotron

We report about feasibility studies of a proposed 3 GHz recyclotron using 20 cell rf structures of spherical shape. As first step a 5 cell Nb cavity was fabricated by deep drawing and electron beam welding. Only chemical etching was used as global surface treatment. After detection of the breakdown location by a resistor network we applied a local mechanical grinding technique. An accelerating field of Eacc= 5.6 MV/m at Qo = 3.2·109 was reached after two mechanical treatments. These results slightly exceed the design values of the recyclotron. No multipacting was observed, indicating that the spherical shape effectively suppresses this type of field limitation in multicell structures.