Design And Fabrication Of A Parallel Flat-field Grating Used In Soft X-Ray Region

Flat-field grating (FFG) is the key optical element of grazing incidence flat-field spectrometer due to its advantages of aberration correction, high resolution and flat focal field and thus is conveniently coupled with plane detectors. With the rapid development of plane detectors in recent years, grazing incidence flat-field spectrometer has been widely used in the study of diagnosis of high temperature plasmas. However, FFG tends to have a narrow working waveband in the soft X-ray region. Therefore time dependent signals, consisting of various different wavelengths, cannot be fully analyzed at one time. In order to solve this problem, a new parallel grating flat-field grating (PFFG) containing two FFGs (G1and G2) lying on one substrate with the same working parameters is introduced. The working wavebands of G1and G2are5-25nm and2-5nm, respectively. PPFG can record the instantaneous signals of plasmas with the same entrance slit and the same plane detector. Thus the working waveband of PFFG is the sum of that of two sub-gratings (2-25nm).The main purpose of this thesis is design and fabrication of PPFG. The subject matter of the thesis are as the follows:1. Design and optimization of PPFG. Based on light path function (LPF), the parameters of PFFG were designed and optimized via genetics algorithm. The optimized angle of incidence is88°and the curvature radius of substrate is9245mm. Aberrations were corrected in the whole wavelength range and the parameters for groove density variation were obtained. Based on the rigorous coupled wave theory (RCWT), the groove profile of PFFG was optimized and the diffraction efficiencies of zeroth order light, first order light and second order light were calculated. The comparison of diffraction efficiencies between PFFG and existing commercial FFG was presented.2. Optimization of recording systems of PFFG. Based on the theory of T. Namioka, the parameters of recording systems of PFFG were optimized and the groove density deviation between optimized value and theoretical value was given. The groove density error caused by the optimized recording system of G1and G2is no more than0.4lines/mm and1lines/mm, respectively, which satisfies the requirements of design. The error of every parameter in the recording system was analyzed and sensitive parameters were found out, which provides the theoretical guidance of the construction of practical recording system. Based on ray tracing method, the resolving power of PFFG was calculated. The comparison results between PFFG and existing commercial FFG demonstrate that the resolving power is pretty much the same.3. Fabrication of FFG. The fabrication technology of PFFG and the instrument equipments used during the fabrication were presented. The construction of practical recording systems are introduced and the groove density error caused by building the recording systems were analyzed. The results demonstrate that the precision of recording systems satisfies the requirements of design. As demonstrated above, PFFG consists of two sub-gratings and thus the alignment error of two sub-gratings directly impacted the resolving power of PFFG. The theoretical allowable alignment error is0.366°. The practical alignment progress was presented and the alignment error is within0.234°, which satisfies the theoretical value. The problems during the fabrication of PFFG were also introduced and the solutions were given. The key technological parameters were studied and the groove profiles with detailed parameters were given in each progress. The fabrication results measured by the atomic force microscopy met all the required parameters.4. Measurements of the groove density of PFFG. The measurement systems based on autocollimation diffractometric method and two-wavelength diffractometric method are presented. The detailed measurement progress are introduced and the measurement error were analyzed. The results demonstrate that the groove density error between fabricated value and designed value is no more than±0.5%, which satisfies the requirements of design.The results demonstrated that PFFG can widen the working waveband of FFG in soft X-ray region and also has the same resolving power with existing commercial FFGs. Therefore the spectrometers equipped with the PFFG are more suitable for diagnosis of plasmas with wider waveband in soft X-ray region.