Hefei Light Source Photoelectric Law Bunch Length Measurement System Developed And Applied Research

Longitudinal beam instability constraints of the high beam current and impact the stable operation of the storage ring.The bunch length and longitudinal distribution are the most important indicators to reflect the occurrence of instability and the strength of the instability.Detection of longitudinal instability and analyzing of the impedance characteristics of the ring can be done by measuring the bunch length and longitudinal distribution with the changes of beam current.In addition,a lot of physical quantities depend on the bunch length parameter.Therefore,the authors developed online system to measure longitudinal bunch length and the distribution of the ring.The time structure of the synchrotron light is same as beam signal in the storage ring.Therefore,the bunch length measurement can be done by measuring the optical pulse in time-domain or in frequency-domain.The bunch length and longitudinal distribution measurement system is developed according to this.Performance of the measurement system takes critical impact on the measurement accuracy and reliability of the final results.Lots of research and practice were took to develop a high precision, high flexibility,cost-effective,bunch-by-bunch,online measurement system.The clock module is the core of the whole system.The bunch length of HLS is so small that the value of clock jitter can take great impact on the measurement result.A stable circuit whose jitter is less than five picoseconds was developed and its impact on the measurement of the bunch length can be ignored.In order to achieve bunch-by-bunch measurement in multi-bunch mode,the bunch selection module was developed and it worked very well.Frequency synthesizer module which can generate precise synchronization signal with the beam signal was developed also.It has great flexibility and can produce any signal in the frequency range of 0.05～2.4GHz to mix the high frequencies selected from the beam signal down to low frequencies.Lengthening of the bunch is one of the focuses in this paper.Potential well distortion was got by measuring the variation of the bunch length and longitudinal distribution with the beam current.The broad impedance of the storage ring can also be got from that.The longitudinal microwave instability didn't occur in HLS in single bunch mode when the beam current was in the range of 1mA to 10mA.It is done by measuring the beam energy spread variation with the beam current.The bunch length is much impacted by longitudinal microwave instability and HOM in multi-bunch mode.The measurement of the bunch length variation with beam current was done and the result was analyzed simply.