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To reach fusion conditions and control plasma configuration in ITER, the next step towards establishing nuclear fusion as viable energy source, suitable combination of additional heating and current drive systems is necessary. Among them, two Neutral Beam Injectors (NBI) will provide 33MW hydrogen/deuterium particles electrostatically accelerated to 1MeV; efficient gas-cell neutralisation at such beam energy requires negative ions, obtained by caesium-catalysed surface conversion of hydrogen/deuterium atoms in the ion source. ITER NBI requirements have never been simultaneously attained; so a Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Italy). Experiments will verify continuous NBI operation for one hour, under stringent requirements for beam divergence (

SPIDER is a 100 kV negative hydrogen or deuterium ion beam source, under construction at Consorzio RFX, which will be exploited for assessment and optimization of the RF source that will be employed in the ITER neutral beam injectors. The 6 MW maximum beam power, with power densities up to 8 MW/m2, is mainly absorbed by the water cooled beam dump, which consists of two panels arranged in V-shape, each one composed of 31 actively cooled hypervapotrons (HVs) made of CuCrZr alloy and vertically stacked. The HVs are spaced from each other to allow the free thermal expansions and their profiles are shaped in order to geometrically realise an overlap to intercept the full beam power, preventing any shine-through. The HVs were originally designed with 0.5 mm nominal overlapping, but during the manufacturing of the left panel it was realized that the design tolerances produced errors within ±0.5 mm, so that the actual overlapping between neighbours HVs ranges between 0.0 and 1.0 mm with possible shinethrough during operations. Consequently, the design of the HVs at the right panel has been improved with 1.5 mm nominal overlap to allow safe operation without shine-through and damage on equipment and instrumentation positioned downstream the beam dump. Different solutions have been designed for additional heat transfer elements to be integrated with HVs of the left panel in order to fully intercept the beam power. Finite element models with different geometries of these elements have been analysed and optimised by considering the thermal path, the transient behaviour and the maximum allowable material temperatures. The design solutions have been developed considering technical feasibility and the interfaces with existing structures, actively cooled elements and instrumentation to be mounted on the HVs. Thermal performances and implementation effects have been compared to propose the best solution fixing the shine-through issue.