RF plasma emitter of diagnostic neutral beam injector for the W7-X stellarator

A diagnostic neutral beam injector is being developed in the Budker Institute of Nuclear Physics (Novosibirsk, Russia) for the neutral particle diagnostics in the Wendelstein 7-X stellarator in Greifswald (Germany). The injector ion source is planned to provide a proton beam with a current of up to 10 A, energy up to 65 keV and pulse duration up to 10 s. The plasma emitter provides the ion current density about 120 mA/cm2 at the plane of plasma grid within a circle of 16 cm in diameter. This paper describes the results of the development of the radio-frequency plasma emitter for the ion source. The results of the first experiments with ion beam extraction are presented. Using a four-grid ion optical system permitting the extraction of ion beam current up to 2.4 A, the beam composition was measured by Doppler shift spectroscopy and magnetic mass analyzer. According to the measurement for the fixed beam energy of 50 keV and beam current variable up to 2.4 A, the full-energy beam specie is about 80% for the required ion current density of the plasma emitter.