A Reduced Model for Predicting Fast-ion $D_\alpha$ Light in Tokamaks

The fast-ion $D_\alpha$ (FIDA) diagnostic measures visible photons emitted when energetic deuterium ions undergo charge-exchange reactions with an injected neutral beam [1]. The Doppler shift of the light contains useful information about the fast-ion population but the signal also depends upon many other quantities. A sophisticated forward-modeling code [2] treats all of the relevant atomic physics but there is a need for an approximate code that predicts expected FIDA spectra. This rapid code will be used to remove the FIDA contribution to main-ion $D_\alpha$ spectra, for data mining, and to guide the execution of experiments. Comparisons of simplified models with full calculations are described.\par \vskip6pt \noindent [1] W.W.\ Heidbrink et al., Plasma Phys.\ Control. Fusion {\bf 46} (2004) 1855. [2] W.W.\ Heidbrink et al., Comm.\ Comp.\ Phys.\ (2010) submitted.