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Description
Neutron diagnostics play a crucial role in fusion reactors by providing key information for plasma and fusion control, machine protection and nuclear safety. In ITER, the neutron diagnostics will be exposed to a wide range of neutron energies, from thermal to fast neutrons, and cover a broad neutron flux range, from 10$^6$ to 10$^{14}$ n.cm$^{-2}$s$^{-1}$. They are used to quantify fusion plasma parameters such as neutron emissivity, D-T ion fueling ratio, total neutron yield and fusion power.
Achieving high accuracy during operation necessitates in-situ calibration deploying inside the tokamak neutron generators with neutron energies of 2.45 MeV and 14.1 MeV as in DD and DT plasmas. The neutron generators are equipped with monitoring systems to record the neutron source emissions during the calibration campaign. As demonstrated at JET, precise characterization of the neutron generators and monitoring systems is essential prior to their use in ITER.
The finality of the in-vessel calibration is to determine accurately the detector calibration coefficients that link experimental counts to physical fusion plasma parameters. This involves considering the detector response, machine integration, and plasma neutron source profile. However, full in-situ calibration is challenging due to the size of the ITER tokamak, the limited yield of the neutron generator and the low sensitivity of systems designed for high-fusion power operation. Therefore, a comprehensive calibration strategy that incorporates high-detail neutronic simulations is necessary to minimize uncertainties and improve cross-calibration accuracy. This strategy includes sensitivity studies to address the effect of uncertainties in the geometrical factors of the tokamak environment and in the material nuclear properties.
This work outlines the calibration strategy for the ITER neutron diagnostics, detailing the characterization of individual detectors, the development of detailed neutronic models and the preparations for in-vessel calibration.
