Bunch-by-bunch Diagnostics at the Aps Using Time- Correlated Single-photon Counting Techniques*

Time-correlated single-photon counting (TCSPC) techniques have been used for bunch purity measurement since the Advanced Photon Source (APS) started operations. Recent improvements of the monitor have increased the signal-to-noise ratio and dynamic range to about 100 billion. Resolution improvements of commercial TCSPC components to under 50 ps FWHM allowed us to measure the longitudinal profile of individual bunches in the APS storage ring using a singlephoton avalanche photodiode and a PicoHarp 300 TCSPC unit. Due to its robustness, the system operates continuously and measures the average longitudinal profile of the stored beam, updating the process variables for bunch phases and bunch lengths every 8 seconds. In a third application, using a TCSPC x-ray detector with an xray wire scanner in the monochromatic beam of the diagnostics undulator, measurements of transverse profiles of individual bunches can be completed in less than 4 minutes. It showed that, due to energy losses to the storage ring impedance, the high-current bunch clearly travels a different trajectory from low-current bunches. These bunch-by-bunch diagnostics provide valuable beam information to users performing timing experiments.