Overview of the Neutral Beam Injector for ITER

To reach fusion conditions and control plasma configuration in ITER, a suitable combination of additional heating and current drive systems is required, which includes two Neutral Beam Injectors (NBI) providing a total of 33MW hydrogen/deuterium particles electrostatically accelerated to 1MeV; efficient gas-cell neutralisation at such beam energy needs negative ions, obtained by plasma-assisted caesium-catalysed surface conversion. The source plasma is generated by 8 inductively-coupled drivers (typical electron density of the order of 1018m-3 and electron temperature of 10-20eV at 0.3Pa filling pressure), expanding into a single 2m tall chamber by diffusing through a magnetic filter. Such a filter lowers the electron temperature and density, creating the conditions for the existence of an ion-ion plasma in front of the apertures of the plasma electrode. The beam features depend on the parameters of this plasma. As ITER NBI requirements have never been simultaneously attained, a Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Italy), hosting two devices, which integrate the experience of several research groups worldwide. MITICA will represent the full-scale NBI prototype with 1MeV particle energy obtained by five 200 kV stages in series. SPIDER, with 100keV particle energy, started testing and optimising the full-scale ion source: extracted beam uniformity >90%, negative ion current density (for one hour) and beam optics (beam divergence <7mrad; beam aiming direction within 2mrad).