Extraction of a strongly focusing He+ beam from three-stage concave electrodes for alpha particle measurement system in ITER

A strongly focusing He+ ion beam source equipped with concave multi-aperture electrodes was developed for production of He- through a charge exchange cell. The beam was extracted at a voltage less than 20 kV from 301 apertures distributed in an area of 100 mm φ, and focused at 750 mm distance. The beam current and the beam size of 2 A and 20 mm in diameter, respectively, were achieved with an arc power less than 10 kW. The optimum perveance was obtained at 0.02 A/kV1.5 at the beam energy less than 20 keV which is suitable for the conversion to He- in an alkali vapor cell.