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A beam position monitor for the undulator by using the optical SR monitor technique has been tested in the Photon Factory. A visible SR in far tail of the undurater spectrum is extracted by a water-cooled beryllium mirror. The extraction mirror has a hole in the center for passing through the central peak of the undulator radiation which has an opening angle of 1/gamma. Extracted visible light in large opening angle has exactly same optical axis of the undulator radiation, because of its a far tail of the spectrum of same radiation. We applied focusing system to observe the beam position in the undulator through the optical image of beam. The results show us this method is applicable to monitor a position of beam in the undulator, and gap change of undulator has no effect of beam position monitoring. We can easily measure the angle of visible ray, this method is applicable not only beam position monitor but also monitoring the angular deviation of undulator radiation.
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