Design of input coupler for the superconducting cavity

The present paper describes in detail the design of the co-axial coupler for the high beta (∼0.42) 700 MHz superconducting cavity. It also describes the Kroll-Yu method for the computation of external Q. A copper prototype of cavity and co-axial coupler is fabricated and the measurement of external Q is carried out. A measured cavity resonant frequency was 694.9 MHz. The external Q was measured for the axial coupler by changing the penetration depth from -74.53 to -29.53 mm the external Q was varied from ∼1.03x106 to 1.38x103. The experimental results and the computational results match well.