Neutronics characterisation of the radiation environment for the qualification of magnetic sensors on DEMO

Magnetic sensors are a primary diagnostic tool for plasma control on DEMO, required to measure the local distribution of magnetic fields at multiple positions. As these sensors must be located between the Breeding Blanket (BB) and the Vacuum Vessel (VV), they will be exposed to a harsh radiation environment, with neutron fluences expected to exceed those of ITER by up to an order of magnitude. This paper presents a comprehensive assessment of the radiation environment at the expected locations of magnetic sensors in DEMO, for both the Helium-Cooled Pebble Bed (HCPB) and Water-Cooled Lithium Lead (WCLL) BB configurations. MCNP simulations were performed using heterogeneous reference models of both blankets. Neutron and gamma fluxes, nuclear heat loads, dose rates and displacements per atom were estimated at 60 in-vessel and 60 ex-vessel positions, distributed poloidally around the plasma. The HCPB configuration produces a maximum in-vessel neutron flux of 3.6×10 13 n cm -2 s -1 at the equatorial inboard positions, a factor 18 higher than the WCLL value at the same location. The WCLL maximum in-vessel neutron flux is 8.6×10 12 n cm -2 s -1 , found below the divertor, where the shielding is less effective. The impact of the Magnetic Strip (MS) – a modular structure proposed to integrate the sensors while ensuring compatibility with Remote Maintenance (RM) – was also assessed, through a parametric study in which the radial thickness of the MS envelope was varied between 5 and 11 cm. Overall, the results presented in this paper establish the boundaries of the radiation environment that must be sustained by the magnetic diagnostic front-end components over the plant lifetime, and provide the quantitative neutronics basis for the irradiation testing and qualification program required to advance the Technology Readiness Level (TRL) of the DEMO magnetic sensors from the proof-of-concept stage to full validation in a fusion-relevant environment.