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Measurements of the neutron emission resulting from nuclear fusion reactions provide an abundance of information on the underlying spatial, temporal and energetic distributions of reacting ions and how they are affected by a wide range of MagnetoHydroDynamic (MHD) instabilities.

This thesis focuses on studies of the neutron emission and fast ion physics at the Mega Ampere Spherical Tokamak (MAST), its upgrade MAST-U, the Joint European Torus (JET) and the Divertor Tokamak Test (DTT). In particular, measurements and simulations of neutron emissivity and neutron rates by collimated neutron flux monitor are here discussed and applied to study the properties of the plasma and of the fast ion distribution.

The first part of the thesis describes plasma measurement methods based on neutron diagnostics. In particular, the design of the neutron camera upgrade on MAST-U is here presented and possible outcomes from its future measurements are discussed. MAST and MAST-U, due to their low plasma temperature, are suitable for fast ion studies and in order to relate neutron measurements with the fast ion distributions the weight functions of the neutron camera on MAST are presented. Fast ions behaviour will be studied as well on DTT where the presence of a collimated neutron flux monitor and a Time Of Flight system is envisaged. Their conceptual designs are presented here. Finally, a novel application of neutron flux monitor for measuring the plasma position is discussed and its application on JET is described here. The second part of the thesis introduces the problem with the “neutron deficit” observed on MAST and the approaches used for its resolution such as the Influence Method and the effect of the Guiding Center and Gyro-Orbit modelling on MAST. The forward modelling based on ASCOT/BBNBI, LINE21, DRESS, NRESP has been compared against the same one but based on TRANSP/NUBEAM simulations. The first modelling was used to validate neutron camera measurements on MAST reducing the observed discrepancy to values within the estimated experimental uncertainties, while the second one was used to benchmark DRESS.