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 v i emission arises from excitation processes that, in the absence of neutral-hydrogen-beam heating, occur near the plasma boundary. Distortion to the C v i 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 ${\mathrm{D}}^0$ 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 ${\mathrm{C}}^{5+}$ ions in their ground state as well as charge-exchange collisions between ${\mathrm{C}}^{6+}$ ions and thermal ${\mathrm{D}}^0$ 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 ${\mathrm{D}}^0$ radial profiles. Comparison of the observed relative intensities of the C v i 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 ${\mathrm{D}}^0$ concentrations near the boundary in high-temperature, high-${\mathrm{D}}^0$-concentration recycling plasmas.