The Principle And Development For High Performance Grating Spectrometer In Soft X-ray Range

In addition to promoting the rapid development of advanced light sources such as synchrotron radiation facilities or free electron lasers,X-ray grating spectrometers are widely used to explore various interesting scientific research topics.Acquisition efficiency and resolving power are the two most important targets of the grating spectrometer.The concave variable line spacing grating is the core of a high-performance spectrometer,which can achieve elimination of astigmatism and high resolving power by combining with the specially designed upstream mirrors.In this paper,two grating spectrometers designed in the"water window"band of soft X-ray are introduced,which realizes the elimination of astigmatism or the improvement of resolution.The main work of this paper includes:1.In order to eliminate astigmatism,firstly,the shape of the meridional focusing curve is controlled by optimizing the grating meridional radius and the variable line spacing coefficient D₁ on the concave variable line spacing grating.Then root mean square error is introduced to evaluate and optimize the"flatness"of focusing curve.Assuming that the target resolving power is constant,the correlation between the grating incident angle and the object distance is obtained,and thus the correlation between of inclination angle of the optimal"flat field"and object distance of grating is found.The sagittal focusing curve is optimized to eliminate the astigmatism in spectral imaging.Firstly,the idea of separation of meridional and sagittal source is proposed.It is proved that when a virtual sagittal source is premised,the sagittal focusing curve and meridional one can be overlapped by simultaneously optimizing the sagittal grating object distance and the grating sagittal radius,then the astigmatism is elimination.2.In the realistic design of a astigmatism elimination spectrometer,a concave cylindrical mirror is placed upstream the concave VLSG to build a structure similar to the K-B focusing system to create a virtual sagittal source for the grating.The parameter for 4 systems with different meridional object distances are calculated.Several contributions that effect final resolving power are investigated,including source size,primary aberration?defocus,coma and spherical aberration?and slope error.The resolving power results that obtained from the ray-tracing software is compared to the results that calculated based on the aberration theory,and a good verification is obtained.By observing the ray-tracing results,the sagittal widths of 4wavelengths are uniform,and it is well fit to the sagittal magnification of the system,which proves that the astigmatism in spectral imaging is eliminated.3.Comparing the ideal resolving power of 4 different spectrometer schemes,the conclusion that pre convex mirror is a better choice for enhancing the resolving power is proved.The optical path function method that considering the extra aberration caused by pre-set mirror is also studied.And the ratio of standard deviation of the meridional coordinates of the outgoing rays and the line width of the diffraction beam distributed at the detector is used to calibrate the imaging quality at each specific wavelength,and according to the definition of the standard deviation,the larger of the ratio value,the more discrete the outgoing rays,the worse the imaging;The support vector machine SVM is introduced for data learning and reconstruction of prediction equations,and the nonlinear programming algorithm is used to explore the optimal system parameters.4.In the realistic design of a resolution enhanced spectrometer,the detailed parameters of the mirror-grating system are calculated based on the optimized parameters set.Aberration analysis was also implemented for this optimized system.The effects of several contribution including the size of the light source,primary aberration?defocus,coma and spherical aberration?and slope error were also investigated.The slope errors conditions that need to be met to achieve the ultra-high resolution are proposed.A superposition method of the slope error contribution from different mirrors is proposed.The resolving power results obtained from the ray-tracing software and the aberration theory are also compared and verified,and a resolving power of 100,000 to 200,000 in the"water window"is achieved.The effects on the spectrometer resolution performance if the source was suffered from longitudinal position jitters and emission angle jitters are also examined.5.Using two independent pre-set mirrors,a keV spectrometer is designed in the energy range of 0.6¹.5keV,and the astigmatism elimination and resolution improvement are achieved simultaneously.The tracing result verified the feasibility of using the meridional concave cylinder grating,with the premise of virtual sagittal source;The calculation method of accumulating the slope error factor from multiple pre-set mirrors in the dispersion direction is given.Finally the resolution improvement results of the pre-convex mirror scheme in different size was calculated and compared.