Energetic particles in reactor-relevant plasmas: modelling and validation

Nuclear fusion is a promising future energy source with few carbon dioxide emissions and nearly limitless source of fuel in heavy isotopes of hydrogen. Energetic particles, such as fusion-born alpha particles and neutral beam injected (NBI) fast ions play a vital role in reactor-relevant fusion plasmas, as they are responsible for heating the plasma, but can simultaneously cause localized heat loads and risk of damage on the plasma facing components. In this work, the Monte Carlo orbit-following code ASCOT has been used to simulate fast ions both to validate simulation results with present-day experiments at the JET tokamak, and to predict fast ion losses in next-generation fusion reactors ITER and DEMO. For validation of ASCOT predictions against JET plasmas, synthetic diagnostics were used to compare the simulated fast ion distributions with the neutral particle analyser (NPA) and fast ion loss detector (FILD) measurements. The NPA simulations qualitatively reproduced the experimentally measured slowing-down distributions and fast ion isotope fraction for NBI-injected hydrogen and deuterium ions, while the FILD simulations for fusion product losses were within 10 % of the experimentally observed losses. For predictions in ITER plasmas, simulations with resonant magnetic perturbations showed that including the response of the plasma to the external perturbations is vital, as the response not only affected the magnitude but also the distribution of fast ion losses. For DEMO plasmas, the sensitivity of fast ion losses due to various magnetic perturbations was studied, including the toroidal field ripple and ferritic inserts in various configurations. The design was found to be robust with respect to fast ion confinement and losses. Finally, over the course of this work, a highly parallelized version of the ASCOT code, called ASCOT5, was developed. The new version substantially increased the performance on modern supercomputer hardware as well as improving its maintainability and extensibility.