Modelling and understanding fast particle transport in non-axisymmetric tokamak plasmas

Understanding fast particle confinement is essential for successful operation of ITER and other advanced fusion experiments. Designing and interpreting experiments require that fast particle transport is studied computationally. The test-particle orbit-following Monte Carlo method is well suited for this purpose as long as the simulations are accompanied with rigorous analysis to ensure the results are valid. This thesis presents a high-performance orbit-following code ASCOT5 for the studies of fast particle transport. The code is verified with respect to known analytical results. Furthermore, this thesis introduces tools to supplement the orbit-following simulations that aid in interpreting the results of the fast particle studies. These tools give credibility to the results and they can also be used to decrease the time required to execute the simulations. ASCOT5 was designed to take full advantage of the modern CPU hardware. The code supports MPI, multithreading, and SIMD instructions resulting in a parallelisation on multiple levels. ASCOT5 uses computational resources more efficiently than its predecessor, but physics-wise they are similar with the exception of the adaptive collision operator. The tools to supplement the orbit-following simulations are demonstrated for ITER. This thesis demonstrates how to construct loss maps which are used to connect fast particle losses to the collisionless transport mechanisms. Constructing loss maps directly from the magnetic field structure provides an alternative way to estimate the fast particle losses without orbit-following simulations. ASCOT5 and the loss-map analysis are used to study fast ion confinement in the presence of various magnetic field perturbations. It was found that the plasma response to the ELM control coils introduces a new loss channel which explains the previously observed shift in fast ion divertor loads. Furthermore, it is shown that the radial transport of runaway electrons and fast ions in an externally perturbed field can be modelled as an advection-diffusion process. This result can be utilized to provide the transport coefficients to the orbit-averaged codes and to perform fast estimates on fast ion losses.