Modeling fuel ion orbits during sawtooth instabilities in fusion plasmas

Sammanfattning: An important part of the fusion research program is to understand and control the large number of plasma instabilities that a fusion plasma can exhibit. One such instability is known as the “sawtooth” instability, which is a perturbation in the plasma electric and magnetic fields that manifests itself as periodic relaxations of the temperature and density in the plasma center. The aim of this project was to investigate how the fuel ions in a fusion plasma react to the sawtooth instability. We were able to implement a model of the plasma electromagnetic field during a sawtooth relaxation into an existing code that computes the orbits of the fuel ions in the tokamak magnetic field. To this end, it was necessary to modify the orbit code to allow for non-zero electric fields, and for time-varying fields. In order to validate the new additions to the code, we compared simulated results to analytical ones. The model of the sawtooth electromagnetic fields required for our simulations was set up within a different student project. However, due to unforeseen complications, only the magnetic (not the electric) field contribution was available to us during our project, but once the electric field is available it is straightforward to include in our code. Our simulations did not exhibit any noticeable perturbation to the particle orbit during a sawtooth crash. However, before the electric field contribution is included it is not possible to draw any physics conclusions from these results. Our code could also be used as a foundation for future projects since it is possible (with further implementations to the existing code) to simulate how the spatial profile of the neutron emission is expected to vary during the sawtooth. These simulations can be compared against experimental measurements of the neutron emission profile in order to investigate the accuracy of the sawtooth model under consideration.