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Abstract
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 ∼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 (⪅1020 m−3) that are similar to those of DIII-D.
Highlights
One of the most common forms of plasma heating in magnetic fusion devices is injection of hydrogenic neutral beams
As an initial test of the concept, the spectrometer of the DIII-D charge exchange recombination (CER) spectroscopy diagnostic [22] was shifted from its usual wavelength to the Dα transition
The Balmer-alpha emission probes the portion of the distribution function with velocities close to vn, as in the case of high-energy negative neutral beam injection
Summary
One of the most common forms of plasma heating in magnetic fusion devices is injection of hydrogenic neutral beams. The spatial profile of Balmer-alpha light from injected neutrals is used to measure the deposition of the neutral beams in the plasma [7, 8]; the spectrum is used to detect magnetic [9] and electric fields [10, 11] through Stark splitting, and fluctuations in the emission are related to fluctuations in the electron density [12]. Balmer-alpha light from the thermal ions that charge exchange with an injected beam provides information on the local ion temperature [13,14] and deuterium density [8,15]. This splitting accounts for the ∼1 nm spread of the three peaks in the full-energy Dα line in figure 1
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