IMPROVEMENT OF X-RAY BEAM POSITION STABILITY FOR BL4-2

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. After the start-up and initial commissioning of BL4-2, the position of the x-ray beam at the focal point in the hutch was measured for several days. The measurements showed that the horizontal beam position was stable within 140 microns and the vertical position of the beam moved up to 400 microns with clear diurnal characteristics. Thus, compared to the size of the focused beam (1.2 mm horizontal and 0.3 mm vertical) the horizontal movement is negligible, but the changes in the vertical beam position needed to be addressed. Variations in the pitch of the focusing mirror were identified as the major contributor to the observed beam motion. Local temperature control of the mirror cooling water was installed in early summer 2008 to reduce the temperature variation of the mirror cooling water and thus minimize the possible dependence of the mirror position due to thermal drift. A reduction was indeed observed (to 250 microns) but not to the required level. Current approaches include implementation of electronic active feedback of the mirror pitch motion.