Space-resolved extreme ultraviolet spectrometer for impurity emission profile measurement in Large Helical Device

摘要

A space-resolved extreme ultraviolet (EUV) spectrometer working in 60–400 Å range has been developed to observe impurity emission profiles in core and edge plasmas of Large Helical Device (LHD). A flat focus is made for plane surface detector by using a varied line spacing holographic grating with an angle of incidence of 87°. An excellent spectral resolution of 0.22 Å at 200 Å is then obtained after careful adjustment of the optical components. In the profile measurement of the LHD plasmas, the toroidal resolution is important as well as the radial resolution because the magnetic surfaces of LHD quickly change when the observation chord is tilted at a slightly different toroidal location. Horizontal dispersion is therefore selected for the present spectrometer. As a result, the toroidal resolution of 75 mm is achieved at the plasma position. An enough radial resolution of 10 mm is also obtained at spatial-resolution slit width of 0.2 mm. In order to measure the full radial profile of LHD plasmas the spectrometer is placed at a distance of ∼ 9200 mm away from the plasma center and a backilluminated charge-coupled device with a size of 6.6×26.6 mm2 is set vertical to the horizontal dispersion. Half of the LHD plasma, i.e., ∼ 50 cm, can be measured along the vertical direction at horizontally elongated plasma cross section. A full vertical profile can be obtained by changing the vertical angle of the EUV spectrometer. As an example the full vertical profile of edge C IV (312.4 Å) emission is presented. A wavelength interval of 35–65 Å can be simultaneously observed, which varies according to the wavelength to be measured. A local emission profile of Fe XX (132.67 Å) is also presented after Abel inversion as a typical example of the core EUV emission. Finally, the EUV spectrometer is absolutely calibrated using EUV bremsstrahlung continuum profile in comparison with absolute values of visible bremsstrahlung continuum profile because the bremsstrahlung continuum can be easily detected in high-density discharges of LHD. © 2010 American Institute of Physics Article Outline INTRODUCTION DETAILS OF SPACE-RESOLVED EUV SPECTROMETER PERFORMANCE OF SPACE-RESOLVED EUV SPECTROMETER Spectral resolution Spatial resolution Toroidal resolution OPERATION OF CCD DETECTOR VERTICAL PROFILES OF EUV EMISSIONS Profile examples of EUV spectra in LHD Analysis of EUV emission profiles SUMMARY