  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 Letter, 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 11.41GeV in
ERL mode. The optimization of the ERL operation, the multi-energy electron beam
transport and the XFEL performance improvements are presented.
