Line emission tomography for CDX-U using filtered diodes

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Abstract
Electron density and temperature in the CDX-U low aspect ratio tokamak are too low to allow observation of fast magnetohydrodynamic activity using soft x-ray continuum emission. However, spectroscopic measurements show that extreme ultraviolet (XUV) line emission of intrinsic impurities is bright enough to observe such activity. In addition, a fast monitoring system for local temperature changes in the plasma core is required for planned auxiliary heating experiments. We present a spectrally resolved tomographic system for fast imaging of O VI 2s–np, 2p–nd (n⩾3), C V 1s2–1s2p and C VI 1s–2p XUV transitions. Using this emission, we can study both core and edge MHD activity, while the C VI to C V intensity ratio can indicate local changes in electron temperature. To achieve maximal throughput together with the needed spectral resolution, we use arrays of surface barrier diodes filtered with bandpass elemental filters. Using M edge filters (Zr, Pd, and Ag), we achieve both good discrimination between the above charge states and large transmission at the wavelengths of interest. Preliminary results obtained from CDX-U are presented.

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