Measurements and Modeling of Fast-ion Light From Edge Neutrals*

Fast ions that are expelled to the plasma edge produce bursts of Doppler-shifted fast-ion $D_\alpha$ (FIDA) light when they charge exchange with edge neutrals [1]. Presently, active FIDA diagnostics use injected neutrals to diagnose confined fast ions but passive FIDA measurements could diagnose losses or provide information on the edge neutral density. In a quantitative test of the technique, prompt losses from a modulated beam provide a known source of fast ions. The edge neutral density is inferred by tomographic inversion of $D_\alpha$ and $D_\gamma$ data. The FIDA spectrum is measured by an instrument that does not view the modulated beams. The data are compared with predictions of a new passive FIDA simulation code that is based on our active FIDA simulation code, FIDASIM [2].\par \vskip6pt \noindent [1] W.W.\ Heidbrink, {\em et al.}, Plasma Phys.\ Control.\ Fusion {\bf 53}, 085007 (2011).\par \noindent [2] W.W.\ Heidbrink, {\em et al.}, Commun.\ Comput.\ Phys.\ {\bf 10}, 716 (2011).