Analytical design of a waveguide iris/stub tuning coupler to an overcoupled superconducting cavity

The high RF power test of a superconducting cavity without beam loading requires a variable coupling scheme. For a fixed power coupler and a finite source power, one needs to increase the external Q. A 3-stub tuner placed between the doorknob transition and an iris plate inside of WR975 waveguide has been developed for the SNS cryomodule test facility (CMTF). This paper describes the theoretical analysis, calculation by equivalent circuits and the coupling scale related to the stub tuning and the iris size. A copper model bench measurement and a 3D computer simulation proved the principle and some of the parameters. A factor of 100 over Q_ext = 7 × 10⁵ has been achieved when implemented in the cryomodule test.