| Shanghai Synchrotron Radiation Facility(SSRF)is the only high-performance third-generation synchrotron light source that has been built and put into operation in mainland China.It is mainly composed of three electronic accelerators: a 150 MeV linear accelerator,a 3.5 GeV booster,and a 3.5 GeV electronic storage ring.The beam running in the electronic storage ring is usually composed of bunch trains.The traditional beam diagnosis system measures the average parameters of the beam(such as closed orbit,turn-by-turn transverse position and turn-by-turn longitudinal phase),and obtains the common behavior information of all bunches,which can meet the general commission and operation requirements.If the individual behavior of each bunch can be monitored and analyzed,the operating performance of the light source can be further improved.On the one hand,achieving high-precision bunch-by-bunch transverse position and longitudinal phase measurement can not only better monitor the operating state of the accelerator,but also study beam instabilities.On the other hand,the injection process of the storage ring is a special transient process.Quantitative observation and analysis of the evolution of the three-dimensional(3D)position of each bunch after injection is helpful to evaluate the matching of the injector and the storage ring and optimize the injection performance.Therefore,high-precision bunch-by-bunch 3D position diagnosis technology is a key technical issue that needs to be studied in advance to improve the performance of the existing facility and build the next generation of synchrotron radiation light sources.The thesis discusses the construction process of the entire bunch-by-bunch 3D position measurement system.The main contents include the pickup of the beam signal,the signal acquisition system,and the data processing system.To achieve the two targets of high-precision measurement and online monitoring of the bunch-by-bunch 3D position,SSRF proposes two system solutions: bunch-by-bunch 3D position measurement based on the high sampling rate oscilloscope and the one based on the high-speed acquisition board.The main research contents include:1.Bunch-by-bunch transverse position information was obtained by the deltaover-sum method.Different methods were used to obtain the amplitude of the button electrode signals for the different system solutions,namely interpolation fitting near the crest of the oscilloscope and direct RF sampling of the data acquisition board.2.Bunch-by-bunch longitudinal phase information was obtained by the zerocrossing detection method.Two methods were proposed for the two systems: the correlation function method and the longitudinal delta-over-sum look-up table method.The correlation function method mainly uses all sampling points of the high-sampling oscilloscope to find the best matching phase by pattern matching with a look-up table established by the bunch response function.At the same time,the initial phase and the balanced phase are made up in the data processing,which can be considered as an approximate absolute phase measurement.The construction of the response function is obtained by the waveform reconstruction technology of equivalent sampling.The longitudinal delta-over-sum look-up table method mainly samples two points near the zero-crossing point of each bunch through a data acquisition board.The corresponding phase in the phase look-up table is found based on the sum-difference ratio of the two channels signals.The response function of this method is obtained by delay scanning with the clock signal in single-bunch mode,and data splicing after multiple acquisitions.3.System measurement error of the two systems was analyzed,especially the errors introduced by the crosstalk between bunches.The correlation function method used for data processing of high-sampling oscilloscope solves the problem of timedomain signal processing at high bandwidths.Signal processing is limited only by the system bandwidth.At 6 GHz bandwidth,the crosstalk between bunches is small,so the measurement error of the system is small.It is suitable for the quantitative analysis of the absolute phase difference.However,the bandwidth of the bunch-by-bunch 3D position measurement system based on a high-speed acquisition board is only 1.2 GHz,and there is significant crosstalk between bunches.Therefore,it is difficult to determine the relative phase difference between different bunches,which is suitable for the measurement and analysis of the relative change of the phase of a single bunch.4.Experimental measurements of the 3D bunch-by-bunch position were completed in the users’ operation and the injection transient process of SSRF.Error analysis and resolution evaluation of the measurement results were also carried out.The 3D position oscillation process of non-injected stored bunches and refilled bunches after injection were compared to verify the accuracy of the measurement systems.To analyze the main sources of phase measurement errors,the single-point random measurement and principal component analysis(PCA)method were used to evaluate the resolution of the transverse position and longitudinal phase.Based on the highspeed acquisition system,when the bunch charge is greater than 0.6 nC,the measurement resolution of the transverse position is better than 10 μm,and the resolution of the longitudinal phase is better than 0.8 ps(2.5 mrad).The phase measurement based on the correlation function method by oscilloscope can significantly improve the resolution.The best resolution reached 0.2 ps(0.6 mrad).5.Quantitative analysis and research were performed in the injection transient process.Methods for extracting transverse position and longitudinal phase of the refilled charge were proposed: charge weighted average method and reference bunch comparison method.For the transverse betatron damping oscillations and the longitudinal synchrotron damping oscillations,multiple storage ring dynamic parameters such as betatron amplitude,betatron damping time,synchrotron oscillation amplitude,synchrotron damping time,and initial arrival time of the refilled charge were extracted.Among them,betatron amplitude and synchrotron oscillation amplitude represent the mismatch between the storage ring and the injector,and betatron damping time and synchrotron damping time reflect the dynamic performance of the storage ring.6.The long-term 3D position parameters were monitored in the injection transient process.With the help of the analysis of this unsteady-state process,the evolution process of the storage ring dynamic parameters over time was obtained.Thus,online tracking of the machine status and even early warning of operating risks are realized.7.Other applications of the bunch-by-bunch 3D position measurement system were introduced in this research.The high-precision measurement of the bunch-bybunch 3D position can be used to analyze wake-field information and energy loss factors.The beam experiment was carried out in a special filling mode,and the energy loss factor was obtained by changing the charge of the probe bunch and measuring the phase between the probe bunch and the reference bunch.The wake-field model will be further established to extract wake-field information. |
