Concepts for The Magnetic Design of the MITICA Neutral Beam Test Facility Ion Accelerator

MITICA (Megavolt ITER Injector Concept Advancement) is a test facility planned to be built at Consorzio RFX for the development of the full-size Heating and Current Drive Neutral Beam Injectors for the ITER Tokamak reactor [1]. The core of MITICA is constituted by a RF-driven negative Ion Source and by a multi-grid electrostatic Accelerator, which shall produce a 46A H- ion Beam (or a 40A D- ion Beam) with a specific energy up to 1 MeV. The beam is subdivided in 1280 individual beamlets (16 groups of 16x5 beamets) which are to be extracted, accelerated and neutralized under well-controlled conditions in order to produce a focused 1 MeV, 25 MW Neutral Beam. The magnetic configuration inside the accelerator is of crucial importance for the achievement of the required beam optic quality (with the correction of undesired ion beamlet deflections) and also of a good beam efficiency (with the early deflection of the co-extracted and stripped electrons). Different concepts have been proposed and compared for the magnetic configuration inside the electrostatic accelerator, with the objective of optimizing the ion beam optics and, at the same time, minimizing the heat loads due to co-extracted and stripped electrons. Essentially the work has required the pursuit of the best combination of three magnetic field contributions: o "Filter" field, in the vicinity of the Plasma Grid, reduces the number of coextracted electrons and also affects the Negative Ion production efficiency; o "Suppression" field, produced by permanent magnets in the Extraction grid, deflects the co- extracted and secondary electrons; o "Compensation" field, produced by permanent magnets and also by ferromagnetic material in some of the Accelerator grids, compensates the deflection of the accelerated ions caused by the "Suppression" field and deflects the stripped/secondary electron and the remaining co-extracted electrons as well. The paper presents the magnetic configurations considered, comparing in detail the alternative design concepts and discusses the magnetic and beam optics simulation results, evidencing the advantages and drawbacks both from the physics and engineering point of view.