Thermal and structural analysis of high precision beamline position monitors for synchrotron x-ray beams

The photon beam position monitors (PBPM) on the front end of the 7-GeV Advanced Photon Source (APS) synchrotron facility under construction at Argonne National Laboratory (ANL) are precision devices that measure spatial as well as angular positions of the x-ray beams. There exist two such devices on each APS front end, and they are set approximately 4 m apart. The required spatial resolution of these devices is of the order of a few microns; the required angular resolution is less than a micro-radian. These specifications make the material selection, design and stability of the PBPM very challenging tasks. The PBPM blades, which generate the photo-electrons signal from the x-ray beam, are indirectly cooled to prevent overheating by the x-ray beam. Even then, the blade is subject to severe thermal stresses. Prediction of the long-term material and position stability of the PBPM blade requires very careful heat transfer and stress analyses. In this paper, thermal analyses under the prevailing high heat flux from the incident beam and the corresponding stress analyses for various blade configuration are presented. As a result of these analyses, several options in design were developed and compared, including material selection for the blade and its structural detail