Key Techniques Study On Beam Arrival Time And Bunch Charge Measurement Based On Cavity-type Monitors

Compared to conventional light sources,free electron laser(FEL)has extremely brightness,ultra-short pulse length,superior temporal and spatial coherence.With these characters,FEL is available for the research areas such as magnetic imaging,pump-probe ultrafast chemistry,quantum materials,etc.Since the 21 st century,high-gain FEL facilities have made major technological breakthroughs,which has also stimulated the rapid development of high-gain FEL facilities over the world in recent years.At present,China is also planning to build highgain FEL facilities,including Shanghai soft X-ray FEL devices(SXFEL,under construction)and Shanghai hard X-ray free electron laser devices(SHINE,under construction).For such FEL facilities,high-resolution beam arrival time measurement and high-precision bunch charge measurement are key technical issues to be solved.For the operation of FEL facilities,the basic principle is the interaction between the electron bunch and the seed laser.Thus it is necessary to increase the superposition of these two in three dimensions and measure the electron bunch's position.The horizontal position can be measured using the beam position detector,while the longitudinal direction requires accurate measurement of the elect ron beam arrival time and based on this,the timing of the laser pulse is adjusted.For SXFEL,the resolution of the beam arrival time monitor is required to be at least 100 fs.From the perspective of FEL user experiments,especially for experiments with high time resolution requirements,it is also necessary to accurately determine the beam arrival time.The resolution should be at least equal to the length of the bunch,reaching the order of 100 fs.On the other hand,bunch charge,one of the most important and basic parameters for characterizing the state of the electron beam,should be monitored online during the operation.The accurate measurement of bunch charge is also the fundament of charge feedback,beam loss monitoring analysis,and beam lifetime measurement,as well as the basis of the related interlocking work.Therefore,this topic selects the key technologies of high-resolution beam arrival time and high-precision beam charge measurement as the main research direction.At present,there are two main methods for beam arrival time measurement: electro-optic sampling and RF phase cavity method.The electro-optic sampling method uses the beam signal coupled by the broadband probe to amplitude-modulate the ultra-short laser pulse,thereby determining the beam arrival time information by the amplitude of the laser pulse.This method equipped with high sensitivity and high time resolution,but the related system is complicated,difficult to debug,and costly.The electro-optical sampling method is mainly employed at most FEL facilities in Europe,and the current resolution is preferably up to 6 fs.The RF sampling method uses the narrowband signal coupled by the probe to output the intermediate frequency signal through the mixing and calculates the beam arrival time information by detecting the phase of the intermediate frequency signal.The current best resolution of this method is 13 fs at LCLS.Although the resolution is lower than the former,it has already met the operation requirements of SXFEL,and it has a relatively simpler system structure and low cost,also is convenient to debug.The RF sampling method has been widely used at FEL facilities in North America and Asia,such as LCLS,SACLA,SCSS,PAL-XFEL,etc.This topic will focus on the related techniques for measuring beam arrival time based on the RF phase cavity method.For the measurement of bunch charge,there are many current measurement methods,and many products have been commercialized.Commonly used Beam current monitors(BCM)include Faraday cages,DCCTs,ICTs,and button/strip or cavity beam flow detectors.The Faraday cage is an intercept detector,DCCT is not suitable for the measurement of a single pulse charge.Although ICT can be used for single pulse measurement,the base line is susceptible to interference.The button type/strip type BCM can achieve a higher resolution charge amount relative measurement but has a relatively obvious beam lateral position dependence.The cavity BCM is a narrowband system.Such a system can obtain high resolution by repeating the same physical quantity and repeating the sampling,and there is no obvious lateral position dependence of the beam.Therefore,this topic will also use the cavity probe to measure the charge amount of the bunch.In summary,this paper mainly studies the key techniques of beam arrival time and beam charge measurement based on cavity probe.The specific contents include:Study on beam arrival time/cluster charge measurement scheme;Research and development of design methods for cavity;Research on RF signal acquisition and processing methods;Implementation and beam experiment of beam current arrival time and beam charge measurement system based on cavity monitor at SXFEL;Implementation and testing of bunch charge measurement system based on a cavity monitor at SSRF facility.