Design and first measurements of the fast-ion D-alpha diagnostic at the HL-2A tokamak

The fast-ion D-alpha diagnostic (FIDA) is employed to detect Dα light emitted by neutralized fast ions during neutral beam injection. A tangentially viewing FIDA has been developed for the HuanLiuqi-2A (HL-2A) tokamak and typically achieves temporal and transverse spatial resolutions of ∼30 ms and ∼5 cm, respectively. A fast-ion tail on the red shifted wing of the FIDA spectrum is obtained and analyzed with the Monte Carlo code FIDASIM. Good agreement has been presented between the measured and simulated spectra. As the FIDA diagnostic's lines of sight intersect the central axis of neutral beam injection with small angles, the beam emission spectrum is observed with a large Doppler shift. Thus, tangentially viewing FIDA could detect only a small portion of fast ions with an energy of ≈ 20 ∼ 31 keV and a pitch angle of ≈ -1 ∼ -0.8. A second FIDA installation with oblique viewing is designed to minimize spectral contaminants.