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The Linac Coherent Light Source (LCLS) is being upgraded to the high repetition rate (up to 1 MHz) mode using cryogenic modules. As the key optical element at the X-ray Pump-probe (XPP) beamline, the goal of the large offset double-crystal monochromator (LODCM) for the High Energy (HE) upgrade is to maintain its x-ray beam multiplexing capability at higher average beam power (up to 200W) for the whole hard x-ray operating range of 6-25 keV. The upgraded LODCM system will use an upstream diamond transmission grating to achieve high power beam multiplexing. It will enable the 0th order 'transmission’ from the grating to pass through the XPP hutch. The +1st order beam, which contains about 20% power of the incident beam of the grating, will be monochromatized at the 1st crystal position, then directed to XPP experiments at the 2nd crystal position. Both crystal positions will provide 111 and 220 Si crystals. The 1 st crystals need to be cooled by Liquide Nitrogen to minimize their thermal deformation under heat load. The second crystals will be controlled close to ambient temperature. The temperature difference between the two crystals leads to a lattice constant mismatch. The corresponding difference in Bragg angles is utilized to compensate the angle between the 1 st order beam and 0th order beam (initial beam propagation axis) from the grating splitter, making it possible to maintain the propagation direction of the monochromatized beam exiting the LODCM parallel to the 0th order incoming beam.
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