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Title of the thesis: The effect of magnetic configuration on fast ions in stellarators

Thesis defender: Joona Kontula
Opponent: Professor Raúl Sánchez, University Carlos III in Madrid, Spain
Custos: Professor Mathias Groth, Aalto University School of Science

Nuclear fusion is an enticing option for future energy production. Fusion is inherently safe and non-polluting, and hydrogen fuel is abundant in Earth’s oceans. However, conventional tokamak fusion reactors are hindered by the large electrical current required for their operation. This current causes instabilities in the plasma and requires periodic reactor shutdowns. An alternative design called the stellarator does not require any plasma current. This makes stellarators inherently more stable and capable of continuous operation, which makes them especially interesting for commercial energy production.

In this thesis, we studied the behaviour of fast ions in stellarators. Fast ions are produced from plasma heating and fusion reactions. The goal was to model the effect of magnetic configuration on fast ion orbits. Fast ions and their orbits play a key role in determining many of the properties of fusion reactors, such as heat load distributions and fusion reactivity. The fast ions were studied using the ASCOT orbit-following code, developed in partnership between Aalto University and VTT. This allowed studying effects which are otherwise difficult to isolate in experiments.

The simulations predict that the heat loads caused by neutral beam heating do not pose a risk to components in the Wendelstein 7-X stellarator. The neutron flux from fusion reactions was also calculated, and the flux was deemed sufficient for time-resolved measurements of fusion reactivity. A new method for modeling the electric current driven by fast ions was developed and validated against measurements in the TJ-II stellarator.

The results presented in this thesis support the conclusion that the orbits of fast ions in stellarators are mainly determined by the magnetic field shape. The methods for electric current simulations presented allow for optimizing plasma heating methods so that the shape of the magnetic field is not perturbed. The results and methods presented here are helpful for fusion reactor control and also support predicting fast ion properties in future fusion reactors.

Keywords: Fusion, stellarators, fast ions

Thesis available for public display 7 days prior to the defence at . 

Contact information: joona.kontula@aalto.fi