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Emission intensities and line ratios from a fast neutral helium beam

Ahn, J-W. and Craig, D. and Fiksel, G. and Den Hartog, D.J. and Anderson, J.K. and O'Mullane, M.G. (2007) Emission intensities and line ratios from a fast neutral helium beam. Physics of Plasmas, 14 (8). 083301. ISSN 1070-664X

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The emission intensities and line ratios from a fast neutral helium beam is investigated in the Madison Symmetric Torus (MST) [R. N. Dexter, D. W. Kerst, T. W. Lovell, S. C. Prager, and J. C. Sprott, Fusion Technol. 19, 131 1991]. Predicted He I line intensities and line ratios from a recently developed collisional-radiative model are compared with experiment. The intensity of singlet lines comes mostly (>95 from the contribution of the ground state population and is very weakly dependent on the initial metastable fraction at the observation point in the plasma core. On the other hand, the intensity of triplet lines is strongly affected by the local metastable state (2(1)S and 2(3)S) populations and the initial metastable fraction plays an important role in determining line intensities. The fraction of local metastable states can only be estimated by making use of electron temperature (T-e), electron density (n(e)), and effective ion charge (Z(eff)) profiles as inputs to the population balance equations. This leads triplet lines to be unusable for the investigation of their local plasma parameter dependence. The ratio of singlet lines at 667.8 nm and 492.2 nm (I-667/I-492) as well as the ratio of 667.8 nm and 501.6 nm lines (I-667/I-501) has been investigated for the dependence on T-e and n(e) both theoretically and experimentally. I-667/I-492 shows strong dependence on n(e) with weak sensitivity to T-e. Measurements and predictions agree quantitatively within a factor of 2. There has been no ratio of singlet lines identified to have strong enough T-e dependence yet. The ratios are expected to be reasonably insensitive to the variation of Z(eff). (C) 2007 American Institute of Physics.