Theoretical and experimental studies of negative triangularity tokamak plasmas and their applicability to reactor design

Nuclear fusion is a clean and virtually infinite source of energy. Magnetically confined fusion based on the tokamak is the most promising approach to unleash this energy in the near feature. The current reference scenario is based on the High confinement mode (H-mode), which enables the conditions to achieve terrestrial fusion. However, this scenario is plagued by a few concerning issues that might prevent achieving such a result. In this thesis, we explore an alternative scenario, the Negative Triangularity (NT) configuration, which holds the promise to solve such issues. Using a combination of experiments on the TCV tokamak and local gyrokinetic simulations with the GENE code, we explore the interplay between NT geometry and turbulence. In this thesis, we confirmed with experiments on TCV that NT can achieve the same, or even better performance then a Positive Triangularity (PT) H-mode plasma, without the disadvantages that this regime brings with it. We showed that the improved performance is agnostic to the external heating scheme used, making NT plasmas versatile. Using gyrokinetic simulations, we demonstrated that NT plasmas can strongly stabilize Ion Temperature Gradient (ITG) turbulence as well as Trapped Electron Modes (TEM) turbulence. This is important as a reactor-relevant plasma is typically expected to be dominated by ITG transport. Moreover, using gyrokinetic simulations we explored the interplay between triangularity and aspect ratio in different turbulent regimes. We observed that NT geometry strongly stabilizes ITG turbulence at any aspect ratio, proving to be beneficial in both conventional aspect ratio and spherical tokamaks. On the contrary, TEM turbulence is stabilized by NT only at conventional aspect ratio. We also performed some of the first simulations of highly electromagnetic turbulence in NT plasmas, which is important for a reactor. We observed that electromagnetic microinstabilities can be preferentially destabilized by NT (relative to PT), but this looks to be an issue only for spherical tokamaks. Conventional tokamaks should not be affected by this. However, this study was not limited to observation and analysis of the experimental results and simulations. We developed a theoretical model that can be used to understand and predict the impact of NT on turbulence. This new model has been tested against many simulations and even experiments on TCV, always showing a good predictive power. Finally, we explored the feasibility of novel NT reactor scenarios and compared them to reference H-mode scenarios. The results show strong performance and highlighted the benefit of NT relative to the PT H-mode counterpart. In these ways, this thesis contributes to the theoretical and experimental foundation underlying the NT tokamak configuration and its applicability to a fusion reactor.