Key Technologies Study Of Bunch-by-bunch Beam Position Measurement On The Storage Ring

A high current, high brightness and high stability storage ring is the goal of all high energy accelerator workers. It is an important subject to study the beam instability and to find ways to overcome the instability of beam. In this paper, based on the background of Shanghai Synchrotron Radiation Facility(SSRF), the research of bunch-by-bunch beam position diagnostic technique in the storage ring was carried out. First we investigated the research situation of the bunch-by-bunch beam position on the international, domestic existing accelerator storage ring and analysed the realization scheme of the bunch-by-bunch beam position measurement. Analog operation and then converting to digital signal are the main solutions.The purpose of the research is analysis of the physical laws of the electron beam bunch in the storage ring of SSRF, determining the physical process of bunch-by-bunch beam position measurement in the storage ring and the corresponding measurement scheme, giving the demand of electronic measuring instruments in the measurement process and finally realizing the measurement of the bunch-by-bunch beam position in the storage ring of SSRF with the corresponding beam experiment.We used a wide-band, high sampling rate oscilloscope to directly sample the output signal from the button electrode of beam position detector in the storage ring of SSRF. By setting the trigger condition of the oscilloscope, the original output signal of the detector was collected during the injection process of SSRF when running on Top-up operation mode. The sampling period is obtained by using the high resolution FFT algorithm to extract the RF frequency. The peak value of the waveform is selected as the starting point of sampling. And the signal value of each sampling point is obtained by interpolation method. The sum signal of four channels is used to indicate the charge of each bunch. The difference over sum method is used to calculate the position information of each bunch. Beam experiment shows that this method can be used to analyze the motion of each bunch in the storage ring during the injection process.Based on the principle verification of the measurement of the bunch-by-bunch beam position with the wide-band, high sampling rate oscilloscope, we used the mature commercial acquisition equipment to build the platform of the bunch-by-bunch beam position measurement system. A wideband frequency synthesizer(PLL) is used to obtain the sampling clock which is synchronous with the bunch in the storage ring. A data acquisition equipment based on high performance FPGA/CPU is used to digitalize the beam signal and complete data storage and processing. System resolution test experiment shows that the position measurement resolution ofthe system is better than 10μm. And we observe the multi-bunch instability in the storage ring by the measurement platform.This paper also attempted to develop a dedicated processor for the measurement of the bunch-by-bunch beam position. The selection of each component and the design scheme of the system are given. At present, the preliminary design of the whole system has been completed and the simulation work has been done to determine the correctness of the design. The first PCB has been fabricated. But there are problems in welding process to be further debugged and improved.