Thermo-mechanical design of the ITER Neutral Beam Injector grids for Radio Frequency Ion Source and SINGAP Accelerator

This paper focuses on the thermo-mechanical design of the ITER Neutral Beam Injector plasma, extraction and acceleration grids, considering the Radio Frequency option for the negative ion source and the SINGAP option for the accelerator. The SINGle Aperture -SINgle GAP (SINGAP) accelerator for ITER NBI foresees four grids for the extraction and acceleration of negative ions, instead of the seven grids of the Multi Aperture Multi Grid (MAMuG) reference configuration. The Radio Frequency ion source is a promising alternative to the Arc Driven one, showing very good performances in recent experiments at IPP Garching (D) and having the advantage of reduced maintenance requirements. A specific plasma grid was designed for RF source considering lower heat loads and different operational requirements. In particular the thermo-hydraulic and thermo-mechanical design aims at having all over the plasma grid a precisely controlled temperature to enhance the negative ions yield inside the caesium seeded source. Analytical and numerical (FE) thermo-mechanical analyses were also carried out for the extraction and acceleration grids. Structural verifications were accomplished according to the ITER Structural Design Criteria for In-Vessel Components. Design optimizations were performed in order to limit the misalignment of the grids within the tolerance specified by beam optics requirements (0.4 mm of maximum misalignment between corresponding apertures axes). The optimum cooling parameters under different operational scenarios were identified. The paper describes the design status of the grid system for the RF ion source and discusses in detail the numerical work carried out for thermo-mechanical and fluid dynamic behaviour.