Optimization of SPIDER Grounded Grid segment design

The ITER Neutral Beam Test Facility (NBTF), built at Consorzio RFX in Padova, Italy, hosts two experimental devices aiming to test and optimize the ITER Heating Neutral Beam Injectors (HNBs) before construction and installation at ITER site. A full size ion source called SPIDER, producing a 5 MW D- or H- ion beam with 100 kV acceleration voltage, is already operating since 2018 and a full size prototype of ITER HNB called MITICA, producing a 17 MW D0 or H0 neutral beam with acceleration voltage up to 1 MV, is presently under construction. Inside the SPIDER beam source, the extraction and acceleration system is composed of several grids: the plasma grid (PG), the extraction grid (EG) and the grounded grid (GG). Each grid is divided into four segments and feature 1280 beamlet apertures. The grids are equipped with internal cooling channels where water flows to exhaust the applied heat loads and, for some of them, embedded permanent magnets are foreseen to deflect the co-extracted electrons and to optimize the negative ions extraction and acceleration. During the SPIDER experimental campaigns in 2020 a water leakage occurred on one GG segment. The problem was temporarily solved removing the water from the failed segment and reducing the size of beam extraction and acceleration. Then a new GG segment shall be procured and further structural analyses were carried out to identify the possible reason for failure. The outcome of analyses showed that in some local sharp edge areas even minor defects on the cooling manifolds surfaces could be the cause for a delayed failure after some years of operations. Hence, some design changes were identified in order to reduce the local stresses in these areas and the probability of occurrence of similar failures in the future. The proposed changes for improving the structural behaviour of the segment also affected the cooling system performance and the thermo-mechanical behaviour of the segment; so further verifications were carried out to investigate any possible critical issue or performance degradation caused by the proposed design changes. The stresses, deformations, pressure drops, temperature increase and water velocity in the GG segment were evaluated, taking into account the heat loads applied to the upstream surface of the segment. The updated design of GG segment, finally evaluated in accordance to the original requirements and ITER SDC-IC criteria, was used as a basis for the on-going procurement of a new SPIDER GG segment.