A fast-time-response extreme ultraviolet spectrometer for measurement of impurity line emissions in the Experimental Advanced Superconducting Tokamak

Abstract

A flat-field extreme ultraviolet (EUV) spectrometer working in the 20-500 Å wavelength range with fast time response has been newly developed to measure line emissions from highly ionized tungsten in the Experimental Advanced Superconducting Tokamak (EAST) with a tungsten divertor, while the monitoring of light and medium impurities is also an aim in the present development. A flat-field focal plane for spectral image detection is made by a laminar-type varied-line-spacing concave holographic grating with an angle of incidence of 87°. A back-illuminated charge-coupled device (CCD) with a total size of 26.6 × 6.6 mm(2) and pixel numbers of 1024 × 255 (26 × 26 μm(2)/pixel) is used for recording the focal image of spectral lines. An excellent spectral resolution of Δλ0 = 3-4 pixels, where Δλ0 is defined as full width at the foot position of a spectral line, is obtained at the 80-400 Å wavelength range after careful adjustment of the grating and CCD positions. The high signal readout rate of the CCD can improve the temporal resolution of time-resolved spectra when the CCD is operated in the full vertical binning mode. It is usually operated at 5 ms per frame. If the vertical size of the CCD is reduced with a narrow slit, the time response becomes faster. The high-time response in the spectral measurement therefore makes possible a variety of spectroscopic studies, e.g., impurity behavior in long pulse discharges with edge-localized mode bursts. An absolute intensity calibration of the EUV spectrometer is also carried out with a technique using the EUV bremsstrahlung continuum at 20-150 Å for quantitative data analysis. Thus, the high-time resolution tungsten spectra have been successfully observed with good spectral resolution using the present EUV spectrometer system. Typical tungsten spectra in the EUV wavelength range observed from EAST discharges are presented with absolute intensity and spectral identification.

Effect of ∇B drift on the H-mode power threshold in upper single null plasmas with ITER-like tungsten divertor on EAST

“…This reveals that there are other variables beyond the impurity that affect the L-H power threshold in our experiments. Note that the line emissions of impurity levels inside the pedestal region measured by a fast-time-response extreme ultraviolet 25 are also similar, which is not shown here. The importance of edge neutral particles or recycling to the L-H transition has been realized for a long time.…”

Metallic impurity content behavior during ICRH-heated L-mode discharges in EAST

“…One may find detailed information of the spectrometer in Ref. [18,19]. Lines intensity are systematically normalized by lineaveraged electron density to indicate impurity content.…”

Influence of impurity seeding on the plasma radiation in the EAST tokamak

“…Thus, the AXUV photodiodes are suitable to be used for studying the impurity radiation combined with other spectrometer diagnostics [16,17]. There are several spectrum diagnostic systems on EAST, including visible spectrometer and extreme ultraviolet (EUV) spectrometer [18][19][20]. The flat-field EUV spectrometer with wide wavelength range 20-500 Å is installed in midplane to monitor the impurity's line emissions from light elements (e.g., C, O), medium elements (e.g., Ar, Fe), to heavy elements (e.g., Mo, W) in the core of plasma.…”