Development and Performance Test of a Seya-Namioka Vacuum Ultraviolet Spectroscopy System
Impurity behavior study is very important in magnetic confined fusion research as impurity may cause the dilution of fuel ions,affect the power balance,and degrade plasma performance.Spectroscopic diagnostic is important for impurity measurement and transport study in fusion devices.Spectroscopy in the vacuum ultraviolet(VUV)range offers a useful tool for investigating impurity radiation from low-temperatureareasof edge tokamak plasma.To meet the impurity measurement requirement in fusion research,a Seya-Namioka spectrometer was designed,and the main parts of the spectrometer are adjustable width incident slits,concave gratings,and a detector).The luminous flux of the spectrometer is adjusted through an adjustable width of the incident slit,which is of the linear guide type.The position of the incident slit baffle is adjusted through the linear guide to achieve slit width adjustment.The grating surface is coated with aluminum(Al)and magnesium fluoride(MgF2)to enhance the refractive efficiency.The usable wavelength range of the grating is 50～460 nm,and the optical path optimization design is carried out for the 50～50 nm wavelength range.The spectrometer's detector was chosen as a deeply cooled back-illuminated charge-coupled device(CCD).By turning the grating turntable to rotate the grating and change the diffraction angle,spectral observations in the 50～250 nm range can be achieved.Based on the parameters of the concave grating,the specific optical path was determined,and the relationship between wavelength and grating rotation angle,as well as the line dispersion rate at different wavelengths,was analyzed.According to the theory of concave grating imaging,the spectral resolution of the system was calculated and analyzed.The optimal exit arm was determined to be 205 mm by analyzing the spectral resolution under different exit arms.The effects of incident slit width,pixel size,aberration,and diffraction limit on spectral resolution were analyzed with an exit arm of 205 mm and an incident slit of 20 μm.The main contributions to the spectral resolution are the entrance slit width,pixel size,aberration,and diffraction limit.The diffraction limit has the smallest effect on spectral resolution,which can be ignored.Due to the size of the detector pixel,the width of the exit slit has a great impact on the spectral resolution,which remains at about 0.09～0.10 nm and increases slowly in the 50～250 nm wavelength range.The total spectral resolution is between 0.121 nm and 0.122 nm.The width of the incident slit will be adjustable from 10 to 1 000 μm,depending on the intensity of the incident light.The wider the width of the incident slit,the worse the overall spectral resolution will be.In actual measurement,the luminous flux and spectral resolution should be considered.Wavelength calibration and performance tests were performed by using a low-pressure mercury lamp and microwave plasma light source.The wavelength calibration of the spectrometer can be completed based on the zero-order spectrum and the characteristic wavelength of the mercury lamp(Hg Ⅰ 185 nm).The zero-order spectrum position is defined as the diffraction angle zero position.The angular position of Hg Ⅰ 185 nm in the spectrometer is determined according to the formula.The spectral resolution was 0.124 3 at 185 nm by Gauss fitting of the Hg Ⅰ(185 nm)spectral line,which is close to the calculated value.By comparing the spectral resolution of Hg Ⅰ 185 nm under different exit arms through experiments and theoretical calculations,it was further verified that the instrument achieved the best spectral resolution at the exit-arm length of 205 mm.Further performance tests of the spectrometer were carried out using a microwave plasma discharge light source device.It has been verified that the spectrometer has good detection ability in the 50～250 nm wavelength range,based on observing nitrogen,oxygen,and helium lines emitted by microwave plasma light sources.
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