Vertical bunch-size measurement at LEP using very fast X-ray detectors

Very fast X-ray detectors have been developed for beam diagnostics in the LEP machine (Large Electron-Positron Collider). Picosecond CdTe photoconductors allow the measurement of the vertical dimensions of electron and positron bunches (length 50 ps and vertical size 300 μm) during a single beam passage. Intense synchrotron X-rays emitted by the LEP beams are measured each turn by two sets of detectors. The transverse profile detector and its associated electronics are described in detail. A pulsed bias is used to gate the detector for a period of time as short as 10 ns. The profile is then digitized in 10 μs and saved in memory. Typical results obtained in real time with LEP beams are used to illustrate the performance of the detector. The linear pulse bias provides a new way of simultaneously making single-shot X-ray measurements of the longitudinal and vertical bunch centre of gravity, and the vertical profile.