Hydrogenic fast-ion diagnostic using Balmer-alpha light

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 × 10 19 m −3 , visible bremsstrahlung obscures the fast-ion signal. The intrinsic spatial resolution of the diagnostic is ∼5 cm for 40 keV 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 (⪅10 20 m −3 ) that are similar to those of DIII-D.