Gyrokinetic turbulence and transport in the Mega Ampere Spherical Tokamak

Abstract

In this thesis we consider energy transport due to turbulence in the Mega Ampere Spherical Tokamak (MAST), using a multi-scale framework based on local δf gyrokinetics. Transport is modelled globally by means of local flux-tube simulations at each magnetic flux surface. In view of the evidence indicating that turbulent ion transport can be substantially suppressed by rotation shear, our flux tubes span only electron-gyroradius scales. This both simplifies our physics model and reduces the computational resource requirements to a manageable level. Our electrostatic simulations of a MAST H-mode discharge exhibit turbulent electron heat transport comparable with experiment in the outer core region, and are broadly consistent with the paradigm of an electron temperature profile governed by pedestal height and critical electron temperature gradient. Neoclassical transport dominates the ions, and is also comparable with experiment. The two species are decoupled in the sense that the collisional equilibration between ions and electrons is small compared to the sources across most of the plasma. Focusing on a single flux surface in this region, still using both kinetic electrons and kinetic ions, we find that at early simulation times the heat flux "quasi-saturates" with the turbulence dominated by streamer-like radially elongated structures. However, the zonal fluctuation component continues to grow slowly until much later times, eventually leading to a new saturated state dominated by the zonal component. Simplifying further to an adiabatic ion model (which shows the same slow evolution behaviour), we find that in the final saturated state (which determines the macroscopic energy transport) the electron heat flux is approximately proportional to the collision rate. We outline an explanation of this effect based on zonalnonzonal interactions and a scaling of the zonal damping rate with electron-ion collisionality. Improved energy confinement with decreasing collisionality has previously been observed experimentally in STs, and is favourable towards the performance of future devices, which are expected to be hotter and thus less collisional.</p