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 , 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 AkV 1.5 at the beam energy less than 20 keV which is suitable for the conversion to He - in an alkali vapor cell