Numerical exploration of fast ion distributions in fusion devices and their characterization through various mixes of NBI and ICRH systems in view of ITER operations

Energetic particles (EPs) play a crucial role in magnetically confined fusion plasmas, as they contribute to sustain fusion-relevant temperatures, as well as provide current and torque. EPs can be generated by an auxiliary heating systems, such as the Neutral Beam Injection (NBI) and the Ion Cyclotron Resonance Heating (ICRH) systems, and are also produced intrinsically by fusion reactions, for instance in the form of alpha particles. Their interaction with plasma backgrounds and waves can induce instabilities, leading to enhanced transport, energy losses, and reduced confinement. A detailed characterisation of EP dynamics and their distribution functions is therefore essential to maximise their beneficial effects while mitigating adverse impacts on plasma performance. The aim of this thesis is to characterise the behaviour and distribution functions of EPs in tokamak plasmas, with particular emphasis on populations generated by NBI and ICRH systems and their combined operation. State-of-the-art numerical tools are used to study three major devices: JET, DTT, and ITER, each requiring distinct modelling strategies. Part of this work is performed within the Integrated Modelling and Analysis Suite (IMAS). In JET, simulations are used to validate the modelling of ICRH-accelerated EPs through an updated implementation of the orbit-following Monte Carlo code ASCOT coupled with the RFOF wave–particle interaction operator. For DTT, the analysis focusses on NBI EPs and their impact on plasma performance. In ITER, both NBI and ICRH generated EPs are addressed, with particular attention to synergistic effects relevant to performance optimisation under reactor conditions. These studies support the optimisation of heating systems, assess the applicability of existing modelling tools in reactor-relevant regimes, and contribute to the development of new IMAS-compatible tools.