Synchrotron Radiation Monitoring System at Bepc

The synchrotron radiation by the relativistic particles is a useful and important diagnostic tool to obtain various beam parameters. The visible light of the synchrotron radiation emitted in a bending magnet of the BEPC storage ring is reflected through the window by a watercooled mirror in the vacuum chamber and then directed by a remote controlled mirror onto the optical bench supporting the diagnostic instruments in the dark room. These instruments include a CCD camera providing the image of the beam transverse profile on a TV monitor, a lens system focusing the synchrotron light onto a solid state scanned photodiode arrays to obtain the beam height or width, a streak camera to measure the bunch length for a single pass, and a two-dimensional position sensing photo-detector to monitoring the drift of the beam position. This paper will give the detailed description of the synchrotron radiation monitoring system at BEPC. 1 GENERAL DESCRIPTION The Beijing Electron-Positron Collider (BEPC) was built for the high energy physics experiment and the synchrotron radiation research. The BEPC has been in operation for around 10 years. The synchrotron radiation monitoring system has been put into operation since the startup of the BEPC. The characteristics of the electron and positron beams circulating in the ring, such as the transverse beam size, the bunch length and the beam position, can be measured with the system. Table 1 lists the designing values of some parameters related with the synchrotron radiation monitoring system. The vertical beam size in the table assumes an emittance coupling of 3%. Table 1: Some parameters related with the synchrotron radiation monitoring system of BEPC Parameters Collid. mode SR mode Ring energy (GeV) 1.55 2.0 2.2 Revolution freq. (MHz) 1.247 Natural emitt. (nm-rad) 406 666 76 H/V rms beam size at source point (mm) 1.32/0.42 1.69/0.54 0.58/0.26 Rms bunch length (ps) 360 470 255 Bending radius at source point (m) 10.34 Distance from source point to first mirror (m) 1.75 Natural open angle of light, λ=491 nm (mrad) 2.2 Two BEPC primary flat and optically polished mirrors in the vacuum chamber, one each for the positron and electron beams, are made of Cu coasting with gold. The visible part of the synchrotron light is reflected horizontally through the side window by the water-cooled mirror and then directed vertically by a remote controllable mirror to the optical building where contains an optical table, detector electronics, work space. The third mirror brings the light into the horizontal plane again and to the optical table supporting the optical components comprising the detector system. The newly-built optical table was put into use at the beginning of 1998. The surface area of the optical table is 1.81 x 1.2 m and is more than 5 times of the area of the original optical table. Remote Controllable Mirror Photodiode Array