| As an important optical element, gratings play a key role in the field of spectroscopy field. In recent years, with the development of laser and holographic technology, the grating manufacturing level has got a profound progress. Advanced gratings are produced and have been widely used. Concave varied line space gratings, as one kind of aberration corrected holographic grating, have the ability to diffract and focus different wavelengths along a plane, which facilities the spectrum recording by CCDs and other kind of array detectors. This realizes the fast and full spectrum recording by a single exposure. With the rapid development of array detector technology, the application fields of concave varied line space gratings are expanding quickly. The unique features of concave varied line space gratings will be exploited gradually in the near future.This paper consists three parts:grating2D groove density measuring system, the high accuracy wavelength calibration methods for flat field spectrograph with concave varied line space gratings and the application of concave varied line space grating in a soft x-ray spectrograph which is used as a Tokamak plasma diagnostics tool.Groove density of grating is an important parameter that affects the diffraction and imaging performance of a spectrograph or spectrometer. As a product, a grating should undergo factory test and customer acceptance test to make sure its groove distribution meets the application requirements. By the testing, the groove distribution error in grating manufacturing process can be found and corrected. For grating users, the actual parameters of grating groove density will be used for the final optical system optimization during the installation. Moreover, the accuracy of gratings’ groove density will affect the wavelength calibration directly. Therefore the high-accuracy measurement of grating groove density is of great significance.The grating groove density of concave varied line space gratings varies on the different positions on the grating surface; therefore the measurement of the groove density only on the center or just one-dimensional measurement cannot fulfill the demand. In chapter3, a2-dimension measurement setup for grating groove density is established, which achieves high-accuracy measurement of the groove density at any point of a grating. The diffraction measurement method based on the grating diffraction equation and the Littrow configuration is proposed which has the advantage of smaller measuring error. The groove densities of2gratings are tested. It is first time that the2D grating groove density distribution is measured by diffraction method. It is the first time that the curved grooves in holographic grating are observed by our measuring system. This measurement setup could be used to measuring the curvature of the grooves. The accuracy of measurement is defined as the ratio of the RMS error of grating groove density△N and the nominal groove density N. The measuring accuracy of the groove density is about3.4×10-5in our experiment.The wavelength calibration for a spectrometer/spectrograph is an essential procedure before its use. Wavelength calibration for a flat field spectrograph is to determine the wavelength value and the pixel number of the array detector. The wavelength precision is important in the fields such as Tokamak plasma temperature diagnoses and astronomy observation. In chapter4, a wavelength calibration method is proposed which is applied to a soft X-ray flat field spectrometer (Ref to chapter5). A visible range flat field spectrometer with a concave varied line space grating is developed to study high accuracy wavelength calibration method. A parameter-fitting wavelength calibration method has been proposed. In this new method, the physical parameters (grating groove density and relationships between different optical elements such as incident angle, incident arms, et al) of the spectrometer are included in the wavelength calibration model. Compared to the common-used wavelength calibration method, like polynomial fitting, the proposed wavelength calibration method can reach higher wavelength calibration accuracy. As the calibration model uses the physical parameters of the optical system that means these parameters can be computed by the wavelength fitting process. So we can use the wavelength method to evaluate the installation, d alignment and adjust of the spectrometer system. By way, the proposed wavelength calibration method not only can be applied to concave varied line space grating based spectrograph, but also to other spectrograph adopting plane array detectors.Chapter5presents an example of the concave varied line space grating used in soft x-ray spectrographs. The optical system design, structural design, installation and adjustment are described in details. The spectrometers are developed for the Experimental Advanced Superconducting Tokamak (EAST) for Institute of Plasma Physics of Chinese Academy of Science. It is compact in dimension, has high wavelength resolution, covering wide wavelength range and the spatial resolved ability. The impurities in plasma cause lot of radiation loss, which limits the quality of the plasma and affects the high-quality and quasi-steady-state operation of Tokamak. Spectrometer is an important tool to study the impurities transportation in the plasma. There are two spectrometers to cover the wavelength range of10A-500A and the plasma zone area of900mm is covered in space. One spectrometer for short wavelength range of10A-100A, its spectral resolution is0.06A at35A; another spectrometer covers long wavelength range of50A-500A, its spectral resolution is0.15A at200A. Test results show that both spectrometers reach the same spectral resolution among the similar instruments in the world. At present, both spectrometers have been used as a general diagnostic tool for the plasma impurities study which supports a lot to the research of EAST. |
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