Gyrokinetic simulations of turbulent transport in tokamak plasmas

Doctoral thesis, 2014

With the enormous growth of high performance computing (HPC) over the last few decades, plasma physicists have gained access to a valuable instrument for investigating turbulent plasma behaviour. In this thesis, these tools are utilised for the study of particle transport in fusion devices of the tokamak variety. The transport properties of impurities is a major part of the work. This is of high relevance for the performance and optimisation of magnetic fusion devices. For instance, the possible accumulation of He ash in the core of the reactor plasma will serve to dilute the fuel, thus lowering fusion power. Heavier impurity species, originating from the plasma-facing surfaces, may also accumulate in the core, and wall-impurities of relatively low density may lead to unacceptable energy losses in the form of radiation. In an operational power plant, such as the ITER device, both impurities of low and high charge numbers will be present. This thesis studies turbulent particle transport driven by two different modes of drift wave turbulence: the trapped electron (TE) and ion temperature gradient (ITG) modes. Results for ITG mode driven impurity transport are also compared with experimental results from the Joint European Torus. Principal focus is on the balance of convective and diffusive transport, as quantified by the stationary density gradient of zero flux (“peaking factor”, PF). Quasi- and nonlinear results are obtained using the gyrokinetic code GENE, and compared with results from a computationally efficient multi-fluid model. The results are scalings of PF with the driving background gradients of temperature and density, and other parameters, including plasma shape and sheared toroidal rotation.