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As tokamak operation advances toward long-pulse and steady-state regimes, plasma-facing diagnostics will accumulate increasing nuclear loads over the lifetimes of the facilities. In such environments, the possibility of diagnostic failure due to neutron exposure cannot be excluded. Therefore, the design strategy for DEMO diagnostics has favoured robust front-end components compatible with remote handling. Among the set of diagnostics under study, magnetic sensors are the primary tool for plasma control, required to measure the local magnetic field distribution at numerous poloidal and toroidal locations. In-vessel pick-up coils and Hall sensors must be positioned between the breeding blanket and the vacuum vessel, where they will be exposed to a radiation environment with neutron fluences exceeding those expected in ITER by an order of magnitude or more.
This paper presents a comprehensive neutronics characterisation of the radiation environment at the anticipated locations of in-vessel and ex-vessel magnetic sensors in DEMO, for the Helium-Cooled Pebble Bed (HCPB) and Water-Cooled Lithium Lead (WCLL) blanket configurations. MCNP simulations were performed using heterogeneous reference models of both blankets. Neutron and gamma fluxes, nuclear heating, dose rates, and displacements per atom (dpa) were calculated at 60 in-vessel and 60 ex-vessel sensor positions distributed poloidally around the plasma. The influence of the Magnetic Strip (MS) — a modular structure proposed to integrate the pick-up coils, Hall sensors and cable looms while ensuring compatibility with remote maintenance operations — was also assessed, through a parametric study in which the radial carving of the blanket required to accommodate the sensors was varied between 5 and 11 cm. The results show that carving the back of the blanket leads to a pronounced local increase in the nuclear loads at certain positions. Several structural and functional candidate materials were included in the simulations, enabling a complete characterisation of the radiation fields relevant to the design and integration of in-vessel magnetic diagnostics in DEMO.
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