Development of a double-charge exchange He− source

A dc-mode He− source of the double charge exchange type has been developed for the purpose of application to alpha-particle measurement. Because the optimum energy for the two-step two-electron capture process in a rubidium vapor was reported to be around 6 keV, the essential point of development is extraction of an intense He+ beam at relatively low energy. As the discharge current of the He+ ion source is increased, the focusing property deteriorates, and higher electrostatic field is required for extraction. A He− current of 0.07 mA is continuously obtained from a He+ beam extracted from a 6 mm diameter single hole. The conversion efficiency from He+ to He− is about 2%–3% in the energy region of 5–10 keV.