Hydrogenic fast-ion populations are common in toroidal magnetic fusion devices, especially in devices with neutral beam injection. As the fast ions orbit around the device and pass through a neutral beam, some fast ions neutralize and emit Balmer-alpha light. The intensity of this emission is weak compared with the signals from the injected neutrals, the warm (halo) neutrals and the cold edge neutrals, but, for a favourable viewing geometry, the emission is Doppler shifted away from these bright interfering signals. Signals from fast ions are detected in the DIII-D tokamak. When the electron density exceeds ̃7 × 1019 m-3, visible bremsstrahlung obscures the fast-ion signal. The intrinsic spatial resolution of the diagnostic is ̃5cm for 40keV amu-1 fast ions. The technique is well suited for diagnosis of fast-ion populations in devices with fast-ion energies (̃30 keV amu-1), minor radii (̃0.6 m) and plasma densities (≲ 1020 m-3) that are similar to those of DIII-D
