Shu, D. Anton, J.W.J. Assoufid, L. Bassan, H. Decker, F.-J. Gassner, G.L. Huang, Z. Jansma, W.G. Kearney, S.P. Kim, K.-J. Lindberg, R.R. Marcus, G. Mashrafi, S.T. Nuhn, H.-D. Shi, X. Shvyd'ko, Yu. Tan, T.-F. Toter, W.F. White, M. Zhu, D. JACoW Publishing Geneva, Switzerland 2673-5520 978-3-95450-229-5 10.18429/JACoW-MEDSI2020-TUOA02 English 103-106 FEL cavity vacuum laser electron Contribution to a conference proceedings 2021 2021-10 https://doi.org/10.18429/JACoW-MEDSI2020-TUOA02 https://jacow.org/medsi2020/papers/tuoa02.pdf The concept behind the cavity-based X-ray FELs (CBXFELs) such as the X-ray free-electron laser oscillator (XFELO)* and the X-ray regenerative amplifier free-electron laser (XRAFEL)** is to form an X-ray cavity with a set of narrow bandwidth diamond Bragg crystals. Storing and recirculating the output of an amplifier in an X- ray cavity so that the X-ray pulse can interact with following fresh electron bunches over many passes enables the development of full temporal coherence. One of the key challenges to forming the X-ray cavity is the precision of the cavity mechanical system design and construction. In this paper, we present conceptual design of the cavity mechanical system that is currently under development for use in a proof-of-principle cavity-based X-ray free electron laser experiment at the LCLS-II at SLAC.