Near‐field characterization of hydrogen and helium operation on the TFTR diagnostic neutral beam

An optical multichannel analyzer has been used to measure beam divergence and composition. This measurement is usually performed near the center of the neutralizer or beyond the magnet. In the past, these locations suffered difficult beam composition analysis and low light intensity, respectively. It has been determined that the light emission is relatively independent of neutralizer line density in the near field, allowing near‐field measurements to overcome both difficulties. At optimum perveance, but under conditions of high gas throughput, the helium 1/e divergence angle was measured to be 1.5°. Further investigation found that the divergence decreased with gas throughput down to 1.25°. Minimum divergences for the full‐, half‐, and third‐energy hydrogen components were 1.1°, 1.2°, and 1.4°, respectively. Relative neutral hydrogen particle fluxes available for injection into TFTR are a function of perveance. At maximum perveance, the full‐, half‐, and third‐energy atom fractions were 0.25±0.04, 0.5±0.04, and 0.25±0.05, respectively.