Studies On Timing And Synchronization For Ultra-fast Pump-probe Experiments At SSRF

The study of ultrafast processes has been advanced significantly with the rapid development of picosecond and higher time-resolved linear and nonlinear X-ray spectroscopy recently.“Real-time”detection of temporal process changes,such as atomic/molecular energy states,chemical bonds and so on,makes a better understanding of mechanisms of chemical reactions,biological functions and phase transitions in materials.For instance,X-ray absorption spectroscopy is an essential and widely-used tool for in-site researches of electronic and short-range structural properties,especially under extreme conditions,as high temperature,high pressure or close to solid densities.A third-generation synchrotron radiation light source emits at low natural emittance,and therefore,its insertion devices—undulators may provide extremely high brightness X-ray beams.That enables it to be the typical mainstream platform for picosecond resolution X-ray absorption spectroscopy.At present,the third-generation synchrotron radiation sources in the world have experimental stations dedicated to time-resolved researches.The beamline 32ID aims to nano-tomography and the 7-ID to time-resolved x-ray scattering and spectroscopy at the Advanced Photon Source（APS）in USA.Each of them achieves a time resolution of up to 80 ps.The France synchrotron SOLEIL hosts the time-resolved experiments on materials with photoelectron spectroscopy beamline（TEMPO）,in which time-resolved crystallography and structure of condensed matter experiments can be done with a time resolution of 70 ps.The energy-dispersive X-ray absorption spectroscopy（ID24）beamline of European Synchrotron Radiation Facility（ESRF）in Grenoble,France achieves a time resolution of 100 ps as well.At Shanghai Synchrotron Radiation Facility（SSRF）,several time-resolved beamlines are under commissioning or under construction supported by the SSRF Phase-II Beamline Project,including Dynamics Beamline（BL05U,D-Line）,Fast X-ray Imaging Beamline（BL16U）,and Spatial-resolved and Spin-resolved ARPES and Magnetism Beamline（BL07U）.This dissertation focuses on the architecture design of timing and synchronization for the SSRF time-resolved beamlines and,especially,on realization for the ultrafast pump-probe experiments at the D-Line.It includes mainly that design of the picosecond timing and synchronization system and its application,the pulse characteristics of synchrotron X-ray beam（probe）and femtosecond laser（pump）,design of the ps-resolution synchronization for the pump-probe X-ray absorption Spectroscopy,and theorical and experimental studies of jitter factors which affect the synchronization between the pump pulse and the probe pulse.As a probe,the photon flux,spot size and pulse frequency stability of the X-rays used are keys to a pump-probe experiment.Using the tracing code Shadow,we can get that the D-line can deliver a photon flux of around 10⁶ and 10⁷ phot/pulse/0.1%bw at 7ke V for different single electron bunches with 10 m A and 20 m A respectively,while the focus spots in two cases keep the same,（H×V）1×28)².Stability of the X-ray pulse period,6 ps in RMS,is measured experimentally with an X-ray streak camera,and the X-ray pulse jitter is calculated based on the particle swarm algorithm.This jitter includes both the electron bunch jitter and the jitter caused by optical path difference（OPD）of X-rays.An integrated control system programed on the large-scale control software EPICS is set up to ensure precise control of compensation to the OPD jitter.The laser is used as the pump light,and its working wavelengths are related to experimental samples.Wavelengths 1030 nm,SHG 515 nm,THG 343 nm,and FHG266 nm can emit using nonlinear optical crystal frequency doubling technology.When the laser runs at its fundamental wavelength 1030 nm in a frequency of 200 k Hz,the single pulse energy is 250μJ with a pulse width of 266 fs and a spectral width of 6.68nm.In this case,the jitter of the output pulses is 1 ps,and the laser oscillator jitter3.6×10^-7.Compared with a baseband-distributed timing system,a timing system based on Ethernet PTP technology and event timing scheme has higher accuracy,easier integration and more flexible use.The timing hardware EVO selected has a jitter of 3.4ps,which can be configured to EVG,EVR,FANOUT modes.A five-layer timing transmission network is formed in connection with single-mode optical fibers from the original timing system to end-stations.Long-term temperature drift is studied.A temperature drift compensation system is applied and a noteworthy decrement of the jitter caused by the long-term temperature drift is achieved from 15 ps to 2.7 ps measured by a phase noise meter.There exist three key factors,that is,the X-ray pulse jitter,laser pulse jitter,and synchronization timing system resolution,which dominates the pump-probe temporal synchronization resolution.In consideration to the X-ray jitter of 6 ps,the laser pulse jitter of 1 ps,and the jitter of 3.4 ps of the timing hardware EVO,synchronization of less-than 10 ps should be expected.As a matter of fact,detection triggering jitter exists anyway,but fortunately it can be avoided during the experiment because the jitter keeps below a value in general.We measured the detector used for the pump-probe experiments and found that the jitter is most often below 100 ps.For actual experiments,the detector can be triggered in advance by a reasonable amount,for example 200 ps.That means extra exposure should be added to the required gate timing accordingly.Finally,in view of the key factors affecting the control technology of pump-probe experiments,SSRF has designed the pump-probe experimental schemes.The experiment is based on the single bunch operation mode of the storage ring,where the cyclotron frequency of the single bunch is 694 k Hz.In order to synchronize the laser pulse and detector with it,the laser pulse frequency and the detector trigger frequency must be an integer multiple of 694 k Hz.An X-ray pulse frequency of 173.5 k Hz is therefore adopted,with an X-ray utilization rate of 25%（the pulse is detected once in every four cycles）.The trigger frequency of the detector is also set to 173.5 k Hz,which can ensure the synchronization of the three.Specifically,a 500 MHz RF signal is transmitted to the experiment station through the designed timing system.The station EVR parses the received RF signal,and sends a 500 MHz clock signal to the laser synchronization controller,which is divided by 12 and phase-locked to the modulator;another clock signal is used to trigger the detector.By adjusting the EVR frequency division and delay,synchronization control between the laser pulse,X-ray pulse and detector is finally realized.Based on X-ray streak camera detection technology,the synchronous control scheme is used to realize a phase shift control with an accuracy of10 ps between the X-ray pulse and laser pulse with a wavelength of 266 nm.Therefore,considering the convolution of the various jitter factors outlined above,we have a 10ps precision synchronous control,which meets the requirements of a 30～40 ps precision time-resolved pump-probe experiment.