Effects of profiles on microinstabilities in tokamaks

Turbulent transport of heat and particles significantly degrades the confinement in tokamaks. Whilst the confinement improves in larger devices, these are more expensive and the economic viability of future fusion power plants depends upon understanding turbulence so that operating scenarios can be optimised.

Gyrokinetic models are able to describe the plasma turbulence responsible for transport. The assumption that the equilibrium varies slowly relative to the radial width of the instability is often exploited to reduce the global gyrokinetic system to a local one. The relation between the global and local systems is a key topic in this thesis. It is shown that local solutions can only capture the true global behaviour when freedoms in the system are treated correctly. A procedure to reconstruct the global solution from the local one has been developed and successfully tested.

The spontaneous transition to a regime of high confinement, observed on many tokamaks, is associated with the suppression of turbulence in a narrow region near the plasma edge, known as the pedestal, and is accompanied by edge localised instabilities (ELMs) which can eject large amounts of energy in a short time, damaging the confinement vessel. Understanding the ELM and pedestal behaviour is crucial to predict the performance of future tokamaks, as well as offering insight into techniques to reduce the threat of damage due to ELMs. The application of gyrokinetics to study microinstabilities in the edge region of MAST in the time between two ELMs is presented as part of this thesis. This work finds kinetic ballooning modes to be unstable in the pedestal whilst microtearing modes are unstable in the shallow gradient region towards the core. The transition from MTMs to KBMs at the interface between the two regions has been studied and may play an important role in the pedestal evolution.