Design optimization of 3.9 GHz fundamental power coupler for the SHINE project

The third harmonic superconducting cryomodule is being designed for the Shanghai High repetition rate XFEL and Extreme light facility (SHINE) project, which is under construction. In contrast to the European X-ray Free Electron Laser (E-XFEL) project, the 3.9 GHz cryomodules in the SHINE project will operate in the continuous wave regime with higher radio frequency average power for both cavities and couplers. We propose a 3.9 GHz fundamental power coupler with an adjustable antenna length, for satisfying the SHINE project requirements. Here, we describe the 3.9 GHz fundamental power coupler’s design considerations and power requirements for various operating modes of the SHINE Linac. We also present the results of the radio frequency simulation and optimization, including the studies on multipacting and thermal analysis of the proposed 3.9 GHz coupler.