Abstract
The number density of ground state atoms in the 2mm hydrogen arc (T≈20000 K) and the number density of molecules in the 5 mm hydrogen arc (T≈12000 K) have been measured by means of vuv spectroscopy. These species' are particularly likely to deviate from the equilibrium population and may therefore falsify the hydrogen transport coefficients measured in arc experiments. In the present investigation the optically thick line wings of Ly-α and the H2 molecular spectrum around 1600 A have been analyzed. The results demonstrate that the former assumptions of LTE and PLTE, respectively, are completely justified on and near the arc axis. However, in the outer zones of these arcs a considerable overpopulation of the molecules must be expected as a consequence of radial diffusion.
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