Huang, T.M. Gu, K.X. Ma, Q. Zhai, J.Y. JACoW Geneva, Switzerland 978-3-95450-191-5 10.18429/JACoW-SRF2017-MOPB069 English 216-219 MOPB069 ion cavity coupling vacuum cryogenics Contribution to a conference proceedings 2018 2018-01 https://doi.org/10.18429/JACoW-SRF2017-MOPB069 http://jacow.org/srf2017/papers/mopb069.pdf The main ring cavities of CEPC project are two-cell el-liptical superconducting cavities operating at 650 MHz in CW mode. Each cavity equips with one high power input coupler and each coupler has to deliver at least 300 kW of CW RF power to the beam. A variable coupling from 10⁵ to 2·10⁶ is required to meet different operation modes. Considering the cavities working with high quali-ty factor up to 2·10¹⁰, the coupler assembled with cavity in class 10 clean room is strongly recommended to protect the cavity from contamination. Also, low cryogenic heat loss is one of the important issues for a large scale CW operation machine. Some of the above requirements should be compromise. Therefore, it's a big challenge to design a high power input coupler fulfilling the above requirements simultaneously. A new coupler that employs 75 Ω coaxial line sections, a planar ceramic disk win-dow, a coaxial to waveguide transition and a coupling adjusting actuator has been designed. In this paper, the RF design, thermal stress analysis and preliminary me-chanical design of the coupler are presented.