Effect of magnetic islands on fast ions in Wendelstein 7-X and ASDEX Upgrade

This thesis investigates the effects of magnetic islands on the fast ions in tokamak and stellarator configurations, utilizing numerical modeling and validation against experimental data. Magnetic islands can occur in fusion plasmas due to internal or external magnetic perturbations and generally degrade plasma performance. Fast ions, which can be generated in fusion experiments by neutral beam injection (NBI), play an important role in plasma heating and current drive. Understanding their behavior is crucial for the development of fusion energy, since fast ions created by fusion reactions must heat the plasma in a fusion reactor. The simulations with fast ions carried out in this thesis are based on the Monte Carlo code BEAMS3D, which is able to simulate particles in three-dimensional magnetic fields. The code was significantly improved and experimentally validated as part of this work. Improvements include the implementation of a more accurate guiding-center equation of motion, inclusion of toroidal plasma rotation effects, and an updated collision operator that accounts for separate contributions from electrons and ions as well as velocity diffusion. The updated BEAMS3D code has been rigorously compared with the NUBEAM code, which is an established fast ion code for tokamak simulations, showing good agreement. Validation using synthetic measurements from the FIDASIM code and experimental Fast Ion D-alpha (FIDA) spectroscopy data from the ASDEX Upgrade (AUG) tokamak confirmed the predictive capabilities. Agreement was found for on- and off-axis NBI, with the greatest relative improvement in agreement achieved by including velocity diffusion. This validation provided a robust framework for numerically investigating the effects of magnetic islands on fast ions and FIDA measurements. Simulations for AUG plasmas showed a local redistribution of the fast ions and a concentration in the center of the island. However, compared to the changes in the background plasma associated with the appearance of the island, the effect of the magnetic islands itself on the fast ions is small. Effect sizes that are clearly measurable with the FIDA system are only achieved for island sizes well above the stable experimentally observed sizes. The predicted FIDA profiles show a decrease in the core fast ion population as well as profile flattening around the island location. The scaling of the losses of fast ions with the island size indicates diffusive transport behavior. For the Wendelstein 7-X (W7-X) stellarator, the investigations showed that the effects of magnetic islands on the confinement of fast ions in W7-X are similar to those in AUG, despite the different geometries. However, the generally low density of fast ions that can be achieved in W7-X requires careful optimization of the experimental conditions for FIDA measurements. If the conditions are sufficiently optimized, the FIDA signals can be expected to show an increase in the center of the island, which is detected by the FIDA system.