Real time display of the vertical beam sizes in LEP using the BEXE X-ray detector and fast VME based computers

Fast X-ray detectors based on CdTe photoconductors have been installed in LEP since the beginning of its operation in 1989. The angular divergence of the high energy photons from the synchrotron radiation (x-rays) and the narrow spacing of the 64 photoconductors of the detector allow a good measurement of vertical beam profiles down to an rms beam size of 300 mm. This paper presents some specific parameters and experimental results of an upgrade program in which the local processing power of the front-end electronics has been increased by a factor 50. Such a powerful tool has allowed a real time display of the time evolution of the vertical beam sizes. An online correlation plot between the electron and positron beam sizes (turn by turn) is also displayed. These online video images are available in the LEP control room and are used in daily operation for luminosity optimisation.