The confinement of fast ions is of paramount importance for future nuclear fusion reactors, such as ITER. Confined fast ions are needed to heat the plasma into fusion-relevant temperatures, while fast ion losses may compromise the integrity of the vacuum vessel. Fast ions are well confined in an axisymmetric tokamak with a high plasma current, but even small deviations from axisymmetry may lead to localized fast ion losses that may compromise the operation of the machine. 

This thesis describes Monte Carlo simulations of fast ion confinement and losses in ITER as well as existing tokamaks. Since fast ions are relatively collisionless, the theory of collisionless orbits of charged particles in tokamak geometry is first discussed. The collisionless orbits are perturbed by the infrequent collisions with the background plasma and by deviations from magnetic axisymmetry, which. To study these orbits in realistic tokamaks, the theory is put into use in the ASCOT code, whose main features are described in the thesis. Finally, as simulations need to be connected with experiments, the most commonly used fast ion diagnostics are described and their connections to simulations discussed. 

The results obtained in this thesis are encouraging for the operation of ITER. The ASCOT code has been benchmarked with several fast ion diagnostics as well as other codes, and satisfactory agreement has been found. The predictive modelling of ITER suggests that the toroidal field ripple will induce significant fast ion losses, but it can be effectively mitigated by ferritic inserts. The perturbation due to the magnetization of the ferritic steel in test blanket modules is not seen as a threat to fast ion confinement. The impact of ELM control coils on fast ion losses needs more study, but it seems likely that the plasma will screen the error field inside the pedestal, preventing the high fast ion losses seen in vacuum models.