Beam position monitor calibration for the Advanced Photon Source

This paper describes the sensitivity and offset calibration for the beam position monitors (BPMs) using button-type pickups in the injector synchrotron, storage ring, and insertion devices of the Advanced Photon Source (APS). In order to reduce the overall offset and to isolate the error (≲100 μm) due to the low fabrication tolerance in the extruded storage ring vacuum chamber, the electrical offset is minimized by carefully sorting and matching the buttons and cables according to the button capacitance and the characteristic impedances of the cable and the button feedthrough. The wire method is used for the sensitivity calibration, position-to-signal mapping, and measurement of resolution and long-term drift (≲1 mV) of the processing electronics. The processing electronics was also tested at Stanford Synchrotron Radiation Laboratory (SSRL) using a real beam, with results indicating better than 25 μm resolution for the APS storage ring. Conversion between the BPM signal and the actual beam position is done by using polynomial expansions fit to the mapping data with absolute accuracy better than 25 μm within ±5 mm square. Measurement of the effect of button mispositioning and mechanical inaccuracy of the extruded storage ring vacuum chamber, including deformation under vacuum, will be also discussed