RF input coupler with a coupling tuner for an RF acceleration cavity

In a storage ring with a high-current beam, RF coupling between an RF acceleration cavity and a waveguide is usually fixed to high overcoupling. In operation at a low beam current, this overcoupling becomes discrepant and a large amount of RF power is reflected from the cavity. In order to dissolve the coupling discrepancy, a new RF input coupler with a coupling tuner has been developed for SPring-8 accelerator. Its coupling factor can be tuned from 1 to 100 or more according to beam loading. This paper describes the ideal RF input coupler by RF simulations and low RF-power tests.