Contributions from ion-atom charge-exchange collisions to the Cvi Lyman-series intensities in the Joint European Torus tokamak

In a multiple-keV-temperature deuterium plasma such as that in the Joint European Torus (JET), C vi emission arises from excitation processes that, in the absence of neutral-hydrogen-beam heating, occur near the plasma boundary. Distortion to the C vi Lyman decrement, such as the Ly-{β}:Ly-{γ}:Ly-{δ} ratio, and enhancement of high series members (around 1s-12p) are observed. In this situation, large influxes of D⁰ atoms are anticipated at the plasma boundary when the plasma is limited by the carbon inner wall. The intensities of the Lyman-series members are interpreted in terms of electronic excitation of C⁵⁺ ions in their ground state as well as charge-exchange collisions between C⁶⁺ ions and thermal D⁰ atoms, either in their ground state or in their excited states (up to the collision limit). The line-of-sight emission is modeled using an impurity-ion transport code requiring knowledge of the electron temperature, electron density, and D⁰ radial profiles. Comparison of the observed relative intensities of the C vi Lyman series Ly-{β}, Ly-{γ}, Ly-{δ}, etc., with the model calculations allows plausible corrections to be made to the effective charge-exchange rate coefficients as a function of the principal quantum number. The observations offer a potential method of measuring local D⁰ concentrations near the boundary in high-temperature, high-D⁰-concentration recycling plasmas.