Realization of a magnetically compensated extraction grid for performance improvement of next generation NBI

n the multi-beamlet, negative-ion based Heating Neutral Beam (HNB) Injectors presently used in fusion research, arrays of permanent magnets are embedded in the Extraction Grid (EG) for the suppression of the unwanted co-extracted electrons. These magnets cause a significant undesired deflection of the negative ion beamlets, with a typical alternate pattern, matching the orientation of the magnet arrays. As a consequence, a delicate adjustment of the profile of the EG apertures has to be introduced in order to compensate for this deflection by the electrostatic means. During the final design of MITICA, which is the full-scale prototype of the ITER Heating Neutral Beam presently under construction in Padova, a new solution has been developed by Consorzio RFX for the compensation of this undesired deflection. This new approach makes use of an additional set of permanent magnets, called Asymmetric Deflection Compensation Magnets (ADCM), also embedded in the EG, with a series of advantages in terms of performances and versatility with respect to the traditional electrostatic compensation. For this reason, this solution has been adopted as the reference for MITICA, ITER and possibly for DEMO NBI. After a thorough validation of the design by different numerical models, this solution will be experimentally tested for the first time on the Negative Ion Test Stand (NITS) at JAEA Naka Fusion Institute within the framework of a scientific cooperation agreement between JAEA and Consorzio RFX. To this purpose, an EG having an ITER-like profile and compatible with NITS accelerator has been designed and constructed at Consorzio RFX. This paper describes the design solutions adopted for this grid, the construction, and the final assembly procedure of the permanent magnets inside the grid.