Spectroscopic studies of carbon impurities in PISCES-A

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Abstract
The graphite used for the limiter of the tokamak reactor produces carbon-containing molecular impurities as a result of the interactions with the edge plasma. The behavior of these molecular impurities has been studied using emission spectroscopy. The present work includes: (1) finding molecular bands and atomic lines in the visible spectral range, (2) studying the breakup processes of the molecular impurities on their way from the source into the plasma, and (3) developing a spectroscopic diagnostic method for the absolute measurement of the carbon influx resulting from graphite erosion. For these, carbon-containing molecules such as CH4, C2H2, C2H4, CO, and CO2 were injected into the tokamak-boundary-like plasma generated by PISCES-A. Spectrograms of these gases were taken. Many useful bands and lines were determined from the spectrograms. The breakup processes of these gases were studied by observing the spatial profiles of the emission of the molecules and their radicals for different plasma conditions. For the absolute measurement of the eroded molecular impurity flux, the photon efficiency of the lines and bands were found by measuring the absolute number of the emitted photons and injected gas molecules. The chemical sputtering yield of graphite by hydrogen plasma was spectroscopically measured using the previously obtained photon efficiencies. It showed good agreement with results obtained by weight loss measurements.

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