Study Of Bunch-by-bunch Beam Size Measurement Technology

Shanghai Synchrotron Radiation Facility(SSRF)is one of the advanced third generation light sources.With a 432 m round synchrontron electron storage ring,SSRF can storage 3.5GeV electron bunches and generate high-quality radiation light for users.Transverse beam size is a key parameter of electron bunches in storage ring for beam quality evaluation.Hence,high-resolution high-speed beam size measurement will offer better performance for accelerator monitoring and will be benefit to study beam instabilities and optimize machine operation.At present,an interferometer system and an x-ray pinhole system were employed for transverse beam size measurement.The two methods can provide an accurate size value,however,they have limitation in high-speed measurement as long exposure time and low reputation rate of CCD camera.In the second phase upgrade project of SSRF,a large number of insertions will be assembled in storage ring.We will meet large problems of beam transverse instabilities,which requires a powerful beam diagnostic tool to study thess problems.At the same time,for the synchrotron accelerator upgrade and future development of diffraction limited ring,it is necessary for optimizing the injection transient process.The furture challenge of beam diagnostic system is to develop a new method for observing and analyzing the injection transient process,which will provide quantitative suggestion parameters for injection optimization,will improve injection quality and reduce the disturbances to users.In this situation,beam instrumentation group of SSRF developed a bunch-by-bunch beam size measurement system to measure turn-by-turn and bunch-by-bunch transverse beam size of storage ring at SSRF.The bunch-by-bunch beam size measurement system is set up based on visible synchrotron light diagnostic platform of SSRF.It is composed of front-end optical imaging,high-speed optoelectronics detector,signal pick-up and conditioning,high-speed signal acquisition and data processing.In optical part,a telescope system is used for the direct and magnified imaing of light source;high-speed optoelectronics detector is multianode photomultiplier array tube manufacted by Hamamatsu Photonics with ultrafast 0.6ns rise time0;multi-channels signal is picked up by an homemade connection board and amplified by high-bandwidth high gain amplifier module also from Hamamatsu Photonics;high-speed data acquisition is achieved by high-bandwidth high sampling rate oscilloscope or high sampling rate commercial digitizers;data processing is including software resampling,PCA optimization and Gaussian fitting for position and size value in Matlab platform.The bunch-by-bunch beam size measurement system was applied for injection transient process observation and analysis.The betatron oscillation of bunch position and size were successfully captured,which was contributed by the kicker filed mismatch and wakefield oscillation of previous bunches.In spectrum of injection,it was obvious to point the horizontal tune peak and the behavior differences between refilled bunches and stored bunches were also analyzed.The results were corresponding to injection result from Beam Position Monitor(BPM)system,which verified the precision and reliability of the system.Furthermore,PCA method had also been applied to analyse oscillation modes of position and size.One of the system application to SSRF operation was injection comparison of different operation periods,for injection event capture and optimization.In conclusion,the bunch-by-bunch beam size measurement system is successfully developed,implemented and applied in machine study of storage ring at SSRF.It achieves bunch-by-bunch beam size measurement within 2ns spread,and firstly captures and studies betatron oscillation injection behavior of storage ringat SSRF,which demonstrates its capability for fast transverse instability studies In next step,the system improvement will contain two parts: higher resolution using more channels and synchronized sampling signal acquisition method;online data processing based on new and effective position and size calculation algorithm and ultrafast onboard computing capability of FPGA,which can be applied for instabilities capturingat SSRF.