Emission intensities and line ratios from a fast neutral helium beam

Abstract

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 (21S and 23S) 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 (Te), electron density (ne), and effective ion charge (Zeff) 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 667.8nm and 492.2nm (I667∕I492) as well as the ratio of 667.8nm and 501.6nm lines (I667∕I501) has been investigated for the dependence on Te and ne both theoretically and experimentally. I667∕I492 shows strong dependence on ne with weak sensitivity to Te. Measurements and predictions agree quantitatively within a factor of 2. There has been no ratio of singlet lines identified to have strong enough Te dependence yet. The ratios are expected to be reasonably insensitive to the variation of Zeff.