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.
Design of a low voltage, high current extraction system for the ITER Ion Source
A Test Facility is planned to be built in Padova to assemble and test the Neutral Beam Injector for ITER. In the same Test Facility the Ion Source will be tested in a dedicated facility planned to operate in parallel to the main 1 MV facility. Purpose of the full size Ion Source is to optimize the Ion Source performance by maximizing the extracted negative ion current density and its spatial uniformity and by minimizing the ratio of co‐extracted electrons. In this contribution the design of the extractor and accelerator grids for a 100 kV, 60 A system is presented. The trajectories of the negative ions, calculated with the SLACCAD code [1], have been benchmarked by a new 2D code (BYPO [2]) which solves in a self consistent way the electric fields in presence of electric charge and magnetic fields. The energy flux intercepted by the grids is estimated by using the Montecarlo code EAMCC [3] and the grids designed according to the constraints set by the permanent magnets and by the cooling channels. The inte...
Experimental study of ion beamlet steering by aperture displacement in two-stage accelerator
The steering of ion beamlets in a two‐stage accelerator has been measured as functions of both aperture displacement and field intensity ratio between extraction and acceleration regions. Three different types of displacement have been investigated; i.e., displacement of (1) plasma grid, (2) gradient grid, or (3) suppressor grid with respect to the other three grids. In each case, the steering angle is found to be linear with the aperture displacement, and independent of final beam energy and beam current. The variation of steering angle with displacement distance is compared with the value predicted by linear theory using thin lens approximation. It is confirmed that they agree well in the case of (3), while they show appreciable differences in the cases of (1) and (2). This is because the lens effect of the gradient grid is weaker than is predicted by the linear theory. The deterioration of beam optics due to steering has not been observed.