Magnetic confinement of plasma inside a tokamak is presently the most promising form of controlled fusion. A key issue for future fusion devices such as ITER is the interaction between the hot plasma and the cold material surfaces. The density control and exhaust of impurities must be effected in a way not causing excessive heat and particle loads. Edge localized modes (ELMs), intermittent bursts of energy and particles, characterize the standard high confinement (H)-mode. In the recently discovered quiescent H-mode (QH-mode), they are replaced by so-called edge harmonic oscillations of a more continuous nature. The QH-mode is obtained only with counter-injected neutral beams, indicating that fast ions may affect the edge stability properties and thus ELMs.
In this thesis, the neutral-beam-originated fast ions in ASDEX Upgrade tokamak are modelled using the orbit-following Monte Carlo code ASCOT. The modelling results include the edge fast ion slowing-down distribution and the surface loads caused by fast ion losses for co- and counter-injected neutral beams, corresponding to H-mode and QH-mode, respectively. The effects of magnetic field ripple, arising from the finite number of toroidal field coils, and radial electric field >Er are included in the analysis. In addition to neutral beam ions, also the relation of surface distribution of tritium and the flux of tritons created in deuterium-deuterium fusion reactions is addressed.
Due to the difference in the direction of the gradient drift, counter-injected beams are prone to higher losses than co-injected beams. This leads to substantial wall loads, but also to higher edge fast ion density. Also the distribution of the fast ions in velocity space is different. The ripple-induced stochastic diffusion increases the losses, thereby increasing the wall load and reducing the density. The orbit width effects, squeezing for counter-injected and widening for co-injected beams, and orbit transitions caused by >Er further increase the losses and wall load. Nevertheless, they also lead to higher edge fast ion density and changes in the velocity distribution. The obtained 4D distribution functions could be used for gaining insight into the roots of the QH-mode by analyzing the stability properties of the edge for the two injection directions
