Magnetic and thermo-structural design optimization of the Plasma Grid for the MITICA Neutral Beam Injector

MITICA is a prototype of the Heating Neutral Beam (HNB) Injectors for ITER, with the purpose of validating the injector design and optimizing its operation. Its goal is to produce a focused beam of neutral particles (H - or D - ) with energy up to 1 MeV and power of 16 MW for 1 hour. MITICA includes an RF Plasma Source for the production of Negative Ions, a multi - stage Electrostatic Accelerator (up to 1 MV and 40 A), a Neutralizer and a Residual Ion Dump. The magnetic field configuration in the injector is crucial for obtaining the required efficiency and beam optics. A suitable magnetic filter field (FF) is produced by the current flowing through the p lasma grid (PG) and related bus - bars. In fact, due to the wide extraction area, it's not possible to obtain a sufficiently uniform FF by means of permanent magnets located on the source edges. The magnetic field distribution in the ion source and accelerat or, and in particular the relative strength in the two regions, have been optimized on the basis of the ITER HNB operational requirements and of the experimental results obtained in RF - driven negative ion sources. An optimal configuration was defined by th e use of an automated design procedure for inverse electromagnetic problems, in combination with a simplified model of the PG, which assumed a uniform current distribution in the PG. The PG current distribution and the uniformity of the resulting magnetic field have been studied by detailed finite element (FEM) models. The introduction of hollow volumes in the thick copper part of the PG among beamlet groups allows a more uniform PG current distribution and thus a more uniform magnetic field in front of the grid. The paper describes in detail the PG geometry optimization procedure and in particular the related magnetic and thermo - structural FEM analyses.