Design And Research Of An Online Spectrometer For The Diagnosis Of Free-electron Lasers In SASE Beamline

Synchrotron radiation technique has developed rapidly in recent years.The range of the spectrum of synchrotron radiation lies from the infrared portion to the hard X-ray portion.The synchrotron radiation has the characteristics of continuous spectrum,high brightness,good collimation etc.Thus it has widely applications in the fields of physics,chemistry,biology,life science as well as material science etc.As the fourth generation of light source instrument,free-electron laser is much better in performance than synchrotron radiation in brightness,coherence and tunable spectrum.At present,several FEL instruments have been built in the worldwide,such as the TESLA in Germany,the LCLS in the US,the FERMI in Italy,SCSS in Japan,Puhang in Korea and Dalian Coherent Light Source?DCLS?and Shanghai soft X-ray free-electron laser facility?SXFEL?in China.SXFEL is the first coherent X-ray light source instrument in China with the wavelength ranging from 2 nm to 10nm.The instrument is based on a 1.5 Ge V C-band linac,including one seeded free-electron laser?FEL?line,one self-amplified spontaneous emission?SASE?FEL line and five experimental stations.The SASE free-electron laser beamline,the beamline for live-cell imaging,is under construction.The energy of the beamline ranges from 103 e V to 1033 e V and the beamline covers two branch lines,including frontend,attenuator,plane mirror PM1,plane mirror PM2?including a variable-line-spacing grating?,monochromator,bendable mirror,Kirkpatric-Baez mirrors and ellipsoidal mirror etc.The beamline which consists of two branch lines is used to meet the need of the experimental stations including biological imaging,surface chemistry and ultrafast physics on the photon flux with the characteristics of small light spot and high pulse energy and realizes the function of the switch between pink beam and monochromatic beam as well as tunable horizontal dimension of light spot according to the specific needs of the experiments.The north branch line consisting of the deflection plane mirrors,an online spectrometer and the Kirkpatric-Baez mirrors?including a plane mirror and two elliptical cylinder mirrors?are used for biological-cell imaging.Owing that the characteristics of the emitted radiation are not as stable as the HGHG?High Gain Harmonic Generation?mode,the measurements of each pulse must be highly reliable and precise so that the experimentalist can use the information make their data more accurate.The same information is useful for machine scientists to improve the performance of the FEL beamline.So far the online spectrometers have been mainly designed for low energies at home and abroad and can not satisfy the requirements for the measurements of resolving power at high enegies without doubt.Thus,an online spectrometer which can be optimized for high energies with a plane mirror and plane variable-line-spacing grating at grazing incidence is designed and presented in this work to monitor each single FEL pulse during the propagation of FEL radiation.Thus,it is necessary to develop a photon diagnostic which is able to measure,shot-by-shot,the energy profile and resolving power of each incoming pulse.to satisfy the requirements of both the users and the machine scientists.An online spectrometer with a plane mirror and plane variable-line-spacing grating at grazing incidence is designed and presented in this work to monitor each single FEL pulse during the propagation of FEL radiation.The method of ray-tracing is used for monitoring the incident radiation to understand the spectral characteristics,and the SHADOW and SRW softwares are applied to study the resolving power,focusing properties of the grating as well as.the influence of slope error on free-electron laser focusing spot.The designed resolving power is spectrometer is compared by the two models.In order to provide further optimization for the choice of gratings,a comparison of resolving powers as well as parameters such as the+1^st order diffraction angle?,the exit arm length r₂ and the tilt angle?between the focal plane and diffraction arm between 2000 l/mm and 3000l/mm gratings at different energies is analyzed and compared in detail.The results show that the energy resolving power of the 3000 l/mm grating can exceed 2.5×10⁴?E/?E?at 1033 e V.In comparison,the energy resolving power is approximately1.5×10⁴?E/?E?for 2000 l/mm grating.Thus the resolving power of the 3000 l/mm grating is much better at high energies?>800 e V?,which is especially useful for the optimization of the spectrometer at high energiers.Meanwhile,B₄C is used as the coating material in the spectrometer to reduce radiation damage to mirrors of the free-electron laser wiht high power.The+1^st order diffraction efficiency of the B₄C-coated grating at 620 e V is able to be approximately 18%when the suitable ratio of the grooving width to the spacing is 0.65 and the grooving depth is 6 nm,which is the best among all the situations.Moreover,during the process of production,optical elements have the slope error which is an important criteria for measuring the surface quality of optical elements.The slope error associated with height error is able to influence the coherent and wavefront of beam and further the final image.As a result,the choice of mirror with appropriate slope error is essential for the design of the beamline.Thus the method of ray-tracing is implemented in the branch line of K-B mirror to trace the X-Ray propagation in the beamline when the actual slope error of mirrors is added.The results show that the actual slope error has little effect on+1^st order light spot of the spectrometer,while the light spot at the end station is significantly affected.According to the ray-tracing results,when the slope error of plane mirrors is 0.1?rad and 0.2?rad and the slope error of elliptical cylinder mirrors is lower than 0.4?rad,the high-quality image of which the size is lower than 4?m is obtained,which is able to meet the needs of imaging in the experimental station.When the actual slope error of the existing mirrors?the slope error of plane mirrors including PM1,PM2and PM7 and elliptical cylinder mirrors including ECM8 and ECM9 are 0.1?rad,0.1?rad,0.2?rad,0.2?rad and 0.2?rad?RMS?,respectively?is added in the simulation,the focusing spot size is 3.3703?m,which is 1.2 times lager than the ideal one.This work has provided comprehensive designing methods and detailed data for the design of diagnostic spectrometers in soft x-ray free-electron lasers,and will be favourable to the design of other similar FEL instruments.