Radio frequency ion source for plasma diagnostics in magnetic fusion experiments

Low-divergent quasistationary neutral beams are often applied in modern mag netic fusion devices as a diagnostic tool providing unique information abou t plasma parameters. The most important requirements of these beams are suf ficiently large current and energy of the particles, so that the beam can p enetrate to the plasma core. Also the duration of the beams must be long en ough, i.e., close to that of a plasma discharge, amounting to at least a fe w seconds for large fusion devices. We developed a neutral beam injector fo r plasma diagnostics in the tokamak TEXTOR-94 which is capable of meeting t hese requirements. The maximum beam energy is 50 keV and the source operate d in hydrogen delivers an ion current of up to 2 A with a pulse duration of up to 4 s. The low divergent beam (similar to 0.5 degrees- 0.6 degrees) is geometrically focused 4 m downstream from the source having a 1/e width of similar to 70 mm at the focal point. The beam can be modulated with a freq uency variable up to 500 Hz. The ion source plasma is produced by a radio f requency discharge in hydrogen or helium. The ion beam is extracted by a fo ur-grid system with 163 single holes. The measured beam parameters were com pared with those predicted by simulations. (C) 2000 American Institute of P hysics. [S0034-6748(00)01910-9].