Improvement in stability of SPring-8 X-ray monochromators with cryogenic-cooled silicon crystals

SPring-8 standard double-crystal monochromators cooled with liquid nitrogen are being improved for providing a stable supply of intense nanometer-focused X-ray beams. The instability originates from the vibration and thermal deformations of the various stages of the monochromators: the former is caused by turbulent flow of the liquid nitrogen, and the latter is mainly due to unwanted cooling from the liquid nitrogen. A low-vibration flexible tube was devised to stabilize the coolant flow by covering the corrugations of the flexible tube with an alumina fiber textile. To achieve thermal insulation, we inserted a machinable ceramic block and a copper plate between the cooled crystal holder and the stages; the temperature of the copper plate was controlled to within ±0.01 °C using a sheet heater and a proportional-integral-derivative current controller. As a result, the vibration was reduced from 1 to 0.15 in terms of the misalignment angle between the two crystals, and a vertical focus size of 230 nm was achieved by demagnification projection of the real light source onto the focal plane. The angular instability due to the thermal deformation was suppressed to a rate of less than 0.2/h. Furthermore, we discuss ongoing improvements for further stabilization.