EVALUATION OF AN OPTICAL BEAM-POSITION-MONITOR SYSTEM WITH CLOSED-LOOP STEERING CAPABILITIES

Imaging the synchrotron source profile onto the entrance slit of a mon ochromator provides a stable and reproducible energy calibration which is independent of the absolute position and drift of the electron bea m. Potential electron-beam motions occurring during a fill result in a loss of flux through the beamline. We have implemented two independen t beam position monitors which can be used as sensors to steer the ver tical entrance mirror in order to maintain a maximum flux through a sp herical grating varied line-spacing monochromator beamline. The system consists of a slotted plate photodiode which intercepts 2 mrad of syn chrotron radiation next to the entrance mirror and a detector utilizin g the photocurrents generated at the jaws of the entrance-slit assembl y. Both monitors have a wide linear response range with a vertical pos ition resolution of < 5 mum. The combination of two independent beam p osition monitors allows an easy check on the mechanical and thermal st ability of the entrance optical system as well as on the reproducibili ty and long-term fluctuations of the electron-beam source during user shifts. We will discuss the performance of the optical beam-position-m onitor system and its implementation as a sensor in a closed-loop feed back system to maintain maximum flux through the beamline.