Feedback system of a liquid nitrogen cooled double-crystal monochromator: design and performances

Abstract : Abstract We report on a new setup of indirect cryogenic cooling system for double-crystal monochromator running on BM30b / FAME beamline at the ESRF (Grenoble, France). This device has been conceived to limit the vibrations on the first diffracting crystal and to maintain it at a constant temperature. These points are crucial to maximize the beamline stability. Moreover, the relative angular position of the second crystal is dynamically adjusted by a piezoelectric transducer coupled with a feedback system in order to always be at the maximum of the rocking curve during a XAS scan. The temperature is stabilized with a 1/100° accuracy with two principal consequences. The energy resolution is close to the theoretical value (E/E=5.6 10-6 for Si(220)) and the precision of the energy positioning is extremely good even if the power load changes. A feedback mechanism allows a permanent and automatic optimisation of the angle between the two crystals of the monochromator. The intensity of the monochromatic beam remains optimized 1) when the intensity of the electron beam decreases in the storage ring and 2) during an energy scan.