Divertor Impurity-Influx Monitor for ITER: Spectral Throughput Measurement on an Optical Prototype for the Upper Port and Optimization of Viewing Chords based on Computerized Tomography

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We are developing a spectroscopic diagnostics system in ultraviolet and visible wavelength regions for monitoring ITER divertor plasmas. An equivalent-size prototype of the optical components for viewing upper port fanarray chords has been assembled as a system to measure spectral throughput, i.e., étendu. Collisional-radiative models for HeI and CIV are used to estimate the emission line intensities of helium ash and carbon impurity ions in a divertor region of a burning plasma. The estimated line intensity of CIV, λ772.6 nm, satisfies ITER requirements for the time resolution of measurement of Ti. A numerical simulation of the computerized tomographic technique for various pairs of viewing fan arrays has been applied to the divertor plasma region to reconstruct a two-dimensional distribution. The optimized pair of viewing fan arrays resolves a model distribution with a reasonable spatial resolution. We measure the reflectance of surfaces of carbon-fiber-composite and tungsten blocks, which make up the plasma-facing divertor target plates and the divertor dome. The reflectance of the surface of the tungsten divertor block is 23% at Hα (λ656.3 nm). A sandblast-processed tungsten surface effectively reduces direct reflectance; the resultant reflectance is less than 0.7%.