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). A review is given of the first 3.5 years of SPIDER operations and of the lessons learnt therein, regarding the current of negative ions at perveance match, the beam divergence and the overall plasma and beam uniformity. Particularly, the strategy to improve the SPIDER performances will be described, which results from detailed diagnostic capabilities combined with a numerical effort, applied to the interpretation of the experimental findings. In parallel, developments on MITICA are underway, including manufacturing of the beam source and the beam line components. Power supplies and auxiliary plants have been installed. Integration, commissioning and tests of the 1MV power supplies are essential for this first-of-kind system, unparalleled both in research and industry field: while insulating tests of high voltage components were successfully completed, during power integrated tests insulation breakdown led to a few component failures. Phenomena have been addressed and solutions for repairing and improving the system were developed.