A superconducting linear accelerator operating in continuous-wave mode could produce X-ray free electron lasers (XFEL) at megahertz repetition rate, with the capability that delivering wide spectral range coherent radiation to multi end stations. In this paper, the energy recovery Linac (ERL) mode is proposed to flexibly control the electron beam energy for a continuous-wave superconducting Linac. Theoretical investigations and multi-dimensional numerical simulations are applied to the Linac case of Shanghai high-repetition-rate XFEL and extreme light facility. The results show that, with ERL operation in the last 25 cryo-modules, the strict requirements on RF power system could be significantly relaxed. And if one exhaust the RF power, the maximum electron beam energy can be enhanced from 8.74 GeV to 11.41 GeV in ERL mode. The optimization of the ERL operation, the multi-energy electron beam transport and the XFEL performance improvements are presented.

超导线性加速器以连续波模式工作可以产生兆赫兹重复频率的X射线自由电子激光（XFEL），并具有向多个终端站提供宽光谱范围相干辐射的能力。本文提出了能量回收直线加速器（ERL）模式，以灵活地控制连续波超导直线加速器的电子束能量。将理论研究和多维数值模拟应用于上海高重复率XFEL和极光设施的直线加速器情况。结果表明，通过在最后25个低温模块中使用ERL，可以大大放松对RF电源系统的严格要求。而且，如果一个人耗尽了RF功率，则在ERL模式下，最大电子束能量可以从8.74 GeV提高到11.41 GeV。ERL操作的优化，