Challenges toward 50 nrad-stability of x-rays for a next generation light source by refinements of SPring-8 standard monochromator with cryo-cooled Si crystals

SPring-8 standard monochromators with liquid nitrogen-cooled silicon crystals are being improved for stable use in future upgraded SPring-8. Our target amount of the vibration is 50 nrad, which is required for the stable supply of intense nanometer-focused x-rays in a typical beamline arrangement. Cryogenic cooling generates vibrations because of two factors: turbulent flow and coolant boiling. Since the latter is caused by low flow rate, further increase in flow rate is required together with a decrease in vibration. For reduction of fluid resistance, a cooling system was modified with large-bore transfer pipes, and the coolant path in the monochromator was branched for both crystals. For reduced vibration, we improved low-vibration flexible tubes, simplified the stage configuration in the monochromator, and increased the rigidity of tilt stages. As a result, no boiling was observed for heat load of more than 400 W. The vibrations were suppressed to approximately 110 nrad for most monochromators. The best stability of 55 nrad was achieved for a monochromator.