Design and test of sub-harmonic cavity's coupler for 150 MeV linac of Shanghai synchrotron radiation facility

With its equivalent RLC circuit, the coupling factor of a sub-harmonic cavity as a function of the cavity’s parameters are derived. This paper presents the design points of the coupler, which is an important part of the sub-harmonic cavity working at 499.654 MHz in the linac of the Shanghai synchrotron radiation facility (SSRF). The structure, the depth into the cavity and the direction of the coupler have been optimized with the three-dimensional finite element method software CST, as well as the coupling factor of the cavity. The cavity has been tested at low RF level. The measurement agrees well with the design values. And some methods on installation and test are given.