Design Optimization For The Soft X-ray Grating Monochromator And The Beamline Of Super-high Energy Resolution

Energy resolution is the key index of a soft X-ray beamline,the level of which will directly determine the performance of the beamline,and this necessitates a systematic study.The objective of this dissertation is,on the basis of the grating monochromator system at soft X-ray spectromicroscopy beamline?BL08U?,to further optimize the engineering design of the grating monochromator and focusing system at Dreamline.The effort is testified by overall energy drifting measured by gas absorption spectroscopy,and super-high energy resolution determination through metal Fermi-edge broadening.The main contents are as follows:The basic parameters of grating monochromator system determining the energy resolution were analyzed.The derived energy resolution of the Dreamline was44,000@867eV,meeting the design requirement of 35,000@867eV.Due to the heat load,the meridian thermal deformation at the plane mirror mounts up to 4?rad for slope error.By means of tuning on optical magnification coefficient(C_ff)and installing an aperture after the monochromator,its impact on energy resolution could be ignored.The parameters are as following:the C_ff value is seven,the meridian slope error of the plane mirror and the grating are 0.2?rad and 0.1?rad,respectively,the vertical size of the source is 49.9?m,and the vertical opening of exit slit is 20?m.The meridian slope error of plane mirror obtained by FEM analysis,resulted in the reduction of the energy resolution to 31,000@867eV,worse than the design requirement.The main factors that cause energy drift were analyzed and optimized,such as energy scanning system of the monochromator,temperature stability of the monochromator and the exit slit etc.,and an overall energy drift of 21meV@867eV was achieved,meeting the design requirement of 24.8meV@867eV.The detailed steps include:i.The rotation error of energy scanning system was analyzed,and it is concluded that the angle error on the driving rod platform was the maximum limiting factor;ii.The self-rotation caused by temperature fluctuation at the monochromator was analyzed,and its maximum temperature difference was found as 0.1 Kelvin;iii.The center height drift of exit slit caused by temperature fluctuation was analyzed,and it is concluded that,Invar alloy with less thermal expansion coefficient should be applied for stand materials.The independent equivalent grating rotation errors for each factor from i)-iii)were 0.1?,0.095?and 0.072?,respectively,which sums up to an overall rotation error as 0.156?.In practice,the stiffness of the driving rod should be increased,the control accuracy for the temperature controlled system?TCS?was tailored as±0.1°C,and the heat produced from equipments in the hutch was taken away by constant temperature cooling water?±0.2°C?.The test results show that the maximum temperature variation achieved 0.20°C,satisfying the design requirement.The transient temperature of the pitch rotation mechanism was analyzed in the toroidal focusing system at BL08U.It is found that,the toroidal mirror will suffer a continued change on its grazing angle with temperature rise of 0.76°C,which caused a continuous drift of the beam spot at the exit slit.The temperature difference at the pitch rotation mechanism became negligible by cooling the toroidal mirror,and the beam spot stabilization time was shortened from 100 minutes or more to about 30minutes.On the basis of this optimization,the Kirkpatrick-Baez?KB?focusing system of the Dreamline was placed behind the exit slit,directly increased the stability of the beam spot at the exit slit.Since the independent of horizontal and vertical focusing,and the greatly reduced focal length,high quality focused beam was provided to the experimental station.The KB focusing system for the Dreamline was developed,a five dimensional high precision adjustment was designed for each mirror to ensure a precise focusing of the beam spot.It is shown that all tested results of the motion mechanism surpassed the design requirements,the resolution and repeatability of the horizontal motion mechanism of the first KB mirror reached 0.6?m and 0.85?m,respectively;and the resolution and repeatability of its pitch angle were 0.4?and 0.5?,respectively.Two methods for determining energy resolution were introduced,including gas absorption spectroscopy and metal Fermi-edge.A series of neon K edge absorption spectra were measured through the exit slit?the maximum limiting factor of energy resolution?varied from 250?m to 20?m.Compared to the related theoretical value,gas absorption spectroscopy works reliably only for the energy resolution less than20,000@867eV,for determining higher energy resolution,the metal Fermi-edge method should be adopted.The gas absorption spectra at neon K-edge was tested by MCP in-situ ionization chamber,and its energy drift was about 15meV@867eV.The Fermi-edge of a metal copper film under T=13K was measured by an electron energy analyzer.The derived energy resolution was 50,000@867eV,which meeting the calculated value and design requirement.