Turbulent transport in tokamak plasmas: linear-, quasi- and non-linear simulations

Doctoral thesis, 2023

An attractive energy source is nuclear fusion with its abundance of fuel, intrinsic safety and limited environmental impact. Although the concept of fusion energy was established in the 1920s, to develop fusion as an energy source has been challenging. The most developed concept for fusion is the tokamak, a torodial shaped chamber where a plasma, a hot ionized gas, is confined with a strong magnetic field. The feasibility and efficiency of the future fusion power plants depend critically on the energy confinement properties of the tokamaks which are mainly determined by micro turbulence.
The turbulent transport is driven by different instabilities in the plasma, especially the Ion Temperature Gradient (ITG) mode, Trapped Electron Mode (TEM) and Electron Temperature Gradient (ETG) mode. The work presented in this thesis focuses on a number of key aspects of turbulent transport using advanced numerical modelling tools.
In today's experiments, measurements have shown the plasma's densities to be peaked towards the centre of the plasma. Research into this peaking has uncovered two key mechanisms, a strong particle pinch from the turbulent transport and a particle source from Neutral Beam Injection which is used to heat plasma. In future tokamaks the source will be comparatively smaller, hence it is important to distinguish which of the two provides the dominant contribution. Which is one of the aspects analysed in the thesis.
From basic considerations, the turbulent transport should exhibit so called gyro-Bohm scaling, i.e. the transport should increase with the ionic mass. However, this is not observed experimentally and the discrepancy is called the isotope effect. Several mechanism has been suggested as the cause, such as collisions, ExB shear, beta-effects, edge effects and contribution of the ETG mode. A number of JET discharges design to study this isotope effect have been analysed to asses the relative importance of these effects,
Calculation of the turbulent transport can be computationally expensive, therefore reduced quasi-linear models that are computationally less intensive have been developed. These models use linear relations between perturbed quantities combined with a saturation rule for the electrostatic potential to determine the turbulent fluxes. A saturation rule adapted to a quasi-linear model has been developed and validated against non-linear gyro-kinetic simulations which are characterized by a high degree of physics fidelity.