Study On Angular Stabilities Of Optics Based On Flexible Hinge Mechanism And Online Monitoring With Interferometry

With the development of advanced X-ray light sources such as diffraction-limited storage rings(DLSRs)and X-ray free-electron lasers(XFELs),on the one hand,highprecision optical components of the beamline are required;On the other hand,mechanical stability also puts forward very high requirements due to the new requirements of experiments such as nano-focusing,long working distance,coherence preservation,and ultra-high stability of photon energy/intensity/position,etc.At present,the angular stability requirements for optical components have reached the order of tens of nanoradians,and the extreme ones have reached more than ten nanoradians.For such high stability requirements,relying on mechanical design can only maintain short-term stability,long-term reliable stability must greatly reduce the influence of various complex vibrations.One of the solutions is to use a technical route combining passive vibration reduction and active optical regulation technology.The optical dynamic feedback control technology is a technique used in beamline construction widely,which can adjust the attitude of the optical components by monitoring the parameters of the X-ray beam or optical components in real time,and control the output direction of the beam to be stable within a certain range.Therefore,the highly stable precision adjustment or motion mechanism,combined with the optical dynamic feedback control technology,will be able to meet the experimental requirements for long-term ultra-high stability of the X-ray beam.For the stability closed-loop control system,the on-line monitoring of the deviation signal with nanoradian precision and the precision mechanical motion with nano-radian precision are the two most critical technologies.The crystal monochromator is one of the core devices in the beamline,and the stability of the crystal attitude has a non-negligible impact on the position stability,energy stability and flux stability of the beam.Therefore,this paper mainly focuses on the on-line monitoring and high-precision motion mechanical structure to achieve nanoradian-level stability in crystal monochromators,including: 1.The heterodyne interferometer for monitoring the attitude of crystals;2.Compliant mechanism of the crystals.The details are as follows:(1)The various measurement error sources of different interference structures of the heterodyne interferometers are studied.Aiming at the large periodic nonlinear error caused by optical structure,an analytical model is established,and the influence of optical elements on periodic nonlinearity in low-fold and high-fold is deduced.The analysis shows that the frequency mixing caused by the polarization properties of corner cube in optical structure is one of the key error sources that limit the measurement accuracy of high-resolution interferometric structure.(2)In order to overcome the shortcomings of the existing technology and reduce the periodic nonlinearity error caused by the polarization properties of the corner cube,a mirror group based on polarization separation is proposed for the first time.Theoretical and experimental results show that the maximum periodic nonlinearity error can be reduced by about 2 orders of magnitude for a common four-fold interferometer by using the mirror group;(3)For the crank-slider mechanism that rotates the crystal,a theoretical model for parameter optimization is proposed using the invariant of crank-slider motion.And according to the requirements of the double-crystal monochromator,a Bragg rotation mechanism with nanoradian resolution in the large rotation angle range is designed,which can realize the linear transmission relationship between the wavelength of the Xray and the displacement of the slider in the large angle range of about 40°.(4)A prototype of the crystal monochromator rotation mechanism based on flexible hinges is developed.The test results show that the deviation between the transmission relationship and the theoretical value of the Bragg rotation axis at each position is less than 10%,the minimum step size that can be resolved is 31.20 nrad,and the stability within 800 seconds is 15.18 nrad.And the adjustment range of the double crystal parallelism motion mechanism is greater than 500μrad,the resolution of the relative angle is 39.51 nrad,and the relative angle stability at different positions is less than 50 nrad.In this paper,the laser interferometric on-line monitoring system and the highresolution compliant mechanism are studied.And the method of reducing the periodic nonlinear error caused by the corner cube through physical design,the established Bragg rotation axis optimization principle and optimization model will have important application significance for realizing high stability technology based on dynamic feedback control and improving beamline performance.