Key Technologies Research And Development Of Cavity Beam Position Monitor For SXFEL

Shanghai Soft X-ray Free Electron Laser(SXFEL)is the first coherent light source in China,which is mainly composed of a photocathode injector,a main accelerator and a twostage high gain harmonic amplifier(undulator).The goal is to achieve 9 nm radiation in the soft X-ray band and verify the feasibility of cascaded high gain harmonic generation.In order to reduce the degradation of FEL output radiation,the orbit of electron and photon beam should be extra close so as to achieve effective energy transmission and conversion from electron and photon beam interaction in the undulator section.To achieve the research purpose,the critical orbit resolution requirement of SXFEL BPM transverse system,should be better than 1?m@500pC?±500?m.In this context,by using the methods of simulation and beam experiments,the research work comprehensively analyzed the factors which affecting the performance of cavity BPM(CBPM)system.And that,the work has constructed a CBPM test platform with relative optimized online signal-processing algorithm and optimization solutions were also be proposed.Based on these works,an integrated online cavity beam position measurement system has been implemented.The followings are the detailed works of the project:1.Study the evaluation method of crosstalk between cavities and the influence of crosstalk on measurement resolution.By analyzing the mechanism of crosstalk between cavities and the characteristics of cavity mode signals(resonant frequency,Q value,position dependence/independence,etc.),the principal component analysis(PCA)and harmonic analysis methods for crosstalk evaluation based on beam conditions are proposed.The verification experiment was completed on the test platform of Shanghai Deep Ultraviolet FEL(SDUV-FEL)and Dalian Coherent Light Source(DCLS),and the practicality of the evaluation method also be verified.At last,the influence of crosstalk on the accuracy and resolution of the beam position measurement is discussed.2.Analyzed the influence of beam angle on the resolution of beam position measurement.Using the path-integral method to analyze the influence of bean trajectory angle and bunch angle on the beam position measurement.The special beam experiment(in combination with corrective iron and adjacent CBPM)was designed and completed to verify the above analysis results.And the experimental results are consistent with the simulation expectations.Based on this,about 100 ?m off-centered operation scheme for CBPM in SXFEL is proposed to ensure that the system resolution meets the requirements.3.Studied the effects of amplitude and phase stability of LO signal on CBPM performance.The influence of amplitude and phase jitter of LO signal on the amplitude extraction accuracy of IF signal was simulated.According to the analysis results,optimized process specifications for LO sources.On the SXFEL,two adjacent phase cavities were used to compare the amplitude and phase extraction accuracy of the signal.Under the condition that other configurations are unchanged,using the optimized LO,the relative charge resolution can be optimized from 0.144% to 0.076%,the beam arrival time resolution from nearly 1ps to 80 fs,and the beam flying time resolution reach 33 fs.The experiment results verified the correctness of the simulation,which also greatly optimizes the performance of the system.4.Analyzed the impact of the stability of timing signal(Trigger/ADC sampling clock)on CBPM system.Based on the analysis results,it is confirmed that when the jitter of the trigger is larger than the sampling clock period of the ADC,the phase extraction of the IF signal will produces a fixed offset that affects the accurate measurement of BAM and the judgment of beam offset.Experiment results were consistent with the simulation and theoretical calculation results.Based on this analysis results,optimized the stability specification requirements of the system timing signal.5.Optimized the FFT algorithm.For the case of insufficient internal resources of FPGA in DBPM electronics,only 512 points of the effective waveform was selected for FFT algorithm processing.A method of fixing the index of the peak frequency was proposed which solved the problem that the demodulated phase has a large error due to the fence effect.6.A test array consisting of three adjacent BPMs was built in the drift section of the SXFEL to evaluate the position resolution of the whole system by the correlation analysis method.At the bunch charge of 500 pC and dynamic range of ±800?m,the position resolution of 880 nm was estimated by experiment,which meet the BPM resolution requirement of better than 1?m@500pC?±500?m from SXFEL.At last,the feasibility of system optimization is analyzed and discussed.According to the quantitative simulation results,three possible optimization techniques are proposed: One is to reduce the frequency of the IF signal to effectively utilize the performance of the ADC;The second is to increase the sampling rate of the ADC to obtain more signal processing gain;And the third is to improve the SNR of the ADC to match the SNR of the IF signal when the SNR of the IF signal is higher than the SNR of the ADC.