Compensation of beamlet deflection by mechanical offset of the grids apertures in the spider ion source

Abstract

Summary form only given. The SPIDER device at RFX has the main goal to test the extraction of negative ions from an ITER size ion source. It is designed to extract 1280 negative ion beamlets and accelerate them up to a 100 kV potential. The negative ion beam at exit and the operating parameters will be carefully measured in order to match the ITER requirements for the NBI ion sources. Inside a negative ion accelerator, there are generally two main factors that can cause deflection of the ion beamlets. Firstly, the ion beamlets can be deflected by the magnetic fields given by the suppression magnets inside the extraction grid. These fields are useful for the suppression of the co- extracted electrons. As a side effect, they cause a difference in deflection between adjacent rows of beamlets (5 mrad for SPIDER), with a consequent ripple effect at the vertical sides of the beam footprint. Secondly, the repulsion between ion beamlets can cause a deflection effect in the outward direction, which is negligible for the beamlets located in the centre of an aperture group and maximum for the ones located at the peripheral apertures, with a consequent increase in the divergence of the whole beam. These two effects are both to be considered highly detrimental for the ITER NBI, as they are foreseen to cause higher heat loads on the ITER NBI neutralizer and decrease the overall beam quality (in terms of aiming and divergence). Hence they should be considered and minimized also for the SPIDER device, where it will be possible to precisely investigate the beamlet footprint using an instrumented calorimeter relatively close to the accelerator exit. This paper presents a design optimization process aiming at minimizing the two described effects. To make this, a mechanical offset of the grounded grid apertures is considered. The OPERA-3d code (Vector Fields Co. Ltd.) is used as main tool for this optimization process, as it can take into account the beamlets repulsion and the interaction between beamlets and grids. This is made by solving the electrostatic Poisson's equation with a finite element approach, to calculate the particle trajectories of the negative ions under the influence of electrostatic fields, magnetic fields and space charge.