Coaxial Cavities for Separated Orbit Cyclotrons

Coaxial cavities are recommended over TM cavities for use in separated-orbit cyclotrons (SOC´s) under 100 MeV primarily because they are smaller and construction tolerances are less stringent. At some energy above 50 MeV, however, the TM cavity1 becomes more efficient. Particles can gain energy in a coaxial cavity with arbitrary spacing between gap centers; however, maximum energy gain occurs for a spacing of 0.5ß¿. In a multi-particle SOC a spacing can be selected to provide acceleration of several different particles. It becomes impractical to define modes in a coaxial cavity relative to the beam motion since the primary TEM mode is defined relative to the direction of wave propagation in the transmission line. Thus TE modes in this paper are likewise defined. The TE modes in a coaxial cavity produce an accelerating voltage which varies with machine radius. Three coaxial cavities have been constructed at ORNL. RF measurements on a 1/4-scale model of a 50-MeV cavity and on the SOCE model have been completed. The 50-MeV prototype cavity is now ready for testing.