Alpha-CHERS: A spectroscopic experiment to detect nonthermal alpha particles on TFTR

A spectroscopic diagnostic for measurement of slowing-down alpha particles is under development and will be implemented for D-T operation of the Tokamak Fusion Test Reactor. The visible 468.6 nm He⁺ line (n=3-4) is excited by charge exchange recombination of alphas with a heating neutral beam (55 keV/amu), and the distortion of the line profile caused by alphas with energies up to 0.5-1.0 MeV is observed. Expected intensities are 0.1%-10% of the bremsstrahlung background for D-T discharges with Q=0.2-1.0. Because the signal is small, a high-throughput optical system is needed to achieve the desired signal-to-noise ratio of 10-100. Vertical and horizontal arrays of sightlines with five spatial channels will be used. Initially, two spatial channels will be coupled via fiber optics to an f/3.8 spectrometer equipped with a low-noise charge coupled device detector, with expansion to 5-10 spatial channels planned for D-T operation.