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Author(s):
Joong-San Koog
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Title:
Development of an Imaging VUV Monochromator in Normal Incidence Region
Date of publication:
July 1996
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Key words:
imaging monochromator, vuv spectroscopy, ray-tracing tokamak, impurity transport
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Abstract:
In high temperature laboratory plasmas used for fusion research, there are considerable spatial variations of plasma parameters from the central part to the plasma edge. Spectroscopic diagnoses in the vacuum ultraviolet (VUV) region, therefore, require instruments which provide spatially resolved measurements for accurate determination of radiative power losses and impurity concentrations. Especially, the spatial distributions of different impurity ions are useful for understanding the impurity transport and confinement properties. In non-axisymmetric plasmas or plasmas with complicated spatial structure, two-dimensional image measurement like human eyes' observation may be attractive. For this purpose, we have developed a two-dimensional imaging monochromator system in VUV wavelength region from 400 to 2000 angstrom. The imaging is achieved with utilizing the pinhole camera effect created by an entrance slit of limited height. The point is that for near normal incidence, the sagittal focusing (perpendicular to the dispersion plane) of a diffracted light by a spherical concave grating is produced at the point outside the Rowland circle. Then, by displacing a two-dimensional detector away from the sagittal focusing point, a one-to-one correspondence between the position of a point on the detector and where it originated in the source is accomplished. The important features of this scheme are two-dimensional imaging which does not require a stigmatic property for focusing and easy fabrication with minor modifications of a commercial normal incidence monochromator. A 1-m normal incidence monochromator with off-Rowland circle mounting is used for this imaging system. The equipped spherical concave grating has the ruling of 1200-lines/mm and is blazed at 1500 angstrom. The detector system consists of two subsystems: a two stages microchannel plate image intensifier assembly and a charge coupled device camera. Ray tracing has been performed to evaluate the imaging properties in the practical geometric configuration and to determine the detector position. The focusing property perpendicular to the dispersion plane was verified by the simple experiment using Hg source and photographic film and compared to the result of ray-tracing. In order to measure the spatial resolution and the relative sen-sitivity on spatial coverage of the imaging system, a stationary arc discharge (TPD-S) was developed as a light source. In order to measure the spatial resolution of the imaging system, an aperture mask has been settled to the position of 40 cm in front of the entrance slit. The measured spatial resolution is about 0.5 mrad and 1 mrad in the dispersion and vertical plane, respectively
, with the entrance slit of 0.1 mm width and height. The spatial sensitivity was also tested using a mercury lamp or the TPD-S apparatus for the VUV wavelength region. The flat
sensitivity was concluded in the direction perpendicular to the dispersion plane. However, it is found that there is a strong non-uniformity of the sensitivity in the direction of the dispersion plane, depending on the observed wavelengths and the blazed wavelength of the used grating. This strong variation in horizontal sensitivity is recognized as the effect of a grating. The usefulness of the imaging system was demonstrated by applying to the experiment of JIPP T-IIU tokamak plasma. The measured distribution of C IV emission at 1548.2 angstrom shows that C IV ion is concentrated in the radial position of sim 20 cm (The limiter radius is ~ 23 cm) and spreads uniformly in the toroidal direction. The differences of the radial distributions which depend on ion species and their ionization stages have been observed on the emission lines of Li-like impurity C IV l548.2 angstrom , N V 1238.8 angstrom and O VI 1031.9 angstrom. Temporal behaviors of the radial distribution have been also measured for O VI emission line at 1031.9 angstrom. We have analyzed and tested the system of imaging monochromator based on pin-hole techniques. The optical characteristics of the system appear promising. In principle, the non-uniformity problem of the sensitivity in the dispersion plane may be solved with an introduction of a non-blazed holographic grating. The interchangeable grating mount equipped with several gratings with different blazed wavelengths is thought to be useful in practice. By incorporating a multilayer coated grating, which can produce high reflectivity at non-grazing angles as Bragg reflectors, the imaging system shown here will be applicable even in the extreme-ultraviolet and soft x-ray region.
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