Evaluation of Distribution Function Models for ICRH-induced Impurity Transport in Tokamaks

Sammanfattning: Fusion power is the utilization of the energy released in nuclear fusion reactions. It has the potential to become an energy source which is more sustainable, safer and cleaner than the primary energy sources today. The fundamental problem for fusion power is energy confinement. Impurity ions are a major source of energy confinement loss in magnetic confinement fusion devices. Hence, impurity transport in fusion plasma is an important field to study. Ion cyclotron resonance heating (ICRH) has been shown both experimentally and theoretically to influence impurity transport in tokamaks. A poloidal asymmetry in the minority ions produces an electric potential, which causes impurity accumulation on the inboard side of the tokamak. Poloidal asymmetry in the impurity density induces a net radial impurity flux over a flux surface. This project has compared the ICRH-induced impurity transport for four approximate minority ion distribution function models with numerical results from the SELFO code. This has been done computationally for JET-like, concentric tokamak geometry with deuterium plasma, hydrogen minority ions, and tungsten impurities. Two models, the so-called bi-Maxwellian and LFS bi-Maxwellian model, are used in existing literature. Two further models are introduced, called the tri-Maxwellian and LFS tri-Maxwellian model. Unlike the bi-Maxwellian models, these models take into account the existence of thermal and fast ions in the minority population. The results show that there are noticeable differences between the different models, in particular when the resonant surface is on the inboard side. The tri-Maxwellian models offer a clear improvement over the bi-Maxwellian models compared with SELFO. However, there are some features in the SELFO results that none of the approximate models predict, this is because the models neglects wide orbits. A possible barrier in the radial transport has also been identified at flux surfaces where the impurity density asymmetry closely resembles the magnetic field strength asymmetry. The LFS bi-Maxwellian model predicts the radial position of the barrier most accurately and reliably compared with SELFO.