Correction of the Coupling of Cesr Rf Cavities to Klystrons Using Three-post Waveguide Transformers

Superconducting CESR RF cavities were designed to handle high RF power delivered to the beam from klystron amplifiers (operating frequency 499.765 MHz). They are strongly overcoupled so that having high Q of the order of 10 they have couplers with external Qext of only 2×10. At a low beam current in the machine, almost full power is reflected from the cavity and goes to the load due to a ferrite circulator mounted between RF cavity and the klystron. With the beam current increasing, the power delivered to the beam goes up and the reflected power goes down. At a certain beam current the reflected power vanishes and the cavity turns perfectly matched to the waveguide power transmission line. If the beam current continues to rise the power reflection reappears and both delivered and reflected powers increase with the beam current.