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The LCLS-II XFEL will be based on a 4 GeV CW SRF linac and will produce x-ray pulses at 1 MHz over the spectral range of 200 to 5,00 eV. The rf gun will be installed and tested in early 2018; cryomodules are being produced at Fermilab and Jefferson lab and shipped to SLAC; undulator segments are being fabricated at LBNL and measured at SLAC. In parallel, a High Energy upgrade will be described which would extend the linac to 8 GeV and increase the spectral range to 12.8 keV.
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Tor O. Raubenheimer (2018) studied this question.