In six decades of research the performance of magnetic confinement fusion (MCF) plasmas has increased by four orders of magnitude. Two crucial challenges to be overcome in upcoming commercial-scale devices are (i) improving performance by reducing the turbulent transport of heat and particles from the plasma core, and (ii) mitigation of energetic eruptions from the plasma edge that will damage plasma-facing components of the vessel.
Since the 1980s an operational mode of fusion devices has been found that achieve (i). This is the high-confinement mode (H-mode) in which plasma turbulence is reduced by the formation zonal flows (ZFs): strongly sheared velocity fields that tears turbulent structures apart. However H-mode plasmas exhibit (ii): quasiperiodic eruptions, termed edge localised modes (ELMs), that carry a significant proportion of the plasma energy. Fortunately ELMs can be somewhat controlled by perturbing the magnetic field at the edge of the plasma, a technique known as resonant magnetic perturbation (RMP).
Recent experiments have found that low-wavenumber oscillations of the plasma, known as geodesic acoustic modes (GAMs), are reduced and eventually elimnated with increased RMP (Robinson et al, PPCF, 2012). GAMs couple strongly to ZFs and are one of the ways of indierctly observing the latter; the implication is that the control mechanism of ELMs adversely affects the turbulence-busting ZFs. It is vital to understand the link between these various effects in order to identify optimal operating regimes.
This PhD project will examine the relationship between RMP, GAMs, ZFs and turbulence. Firstly we will examine data from recent experiments on the MAST tokamak to identify GAMs in various diagnostic channels. Concurrenly, to examine the basic mechanisms involved in the generation of ZFs/GAMs by turbulence, and their interference by RMP, we will develop code to simulate a reduced plasma model in simplified geometry (following Leconte & Diamond, PoP, 2011; Kendl, Phys Plasmas, 2011). Next we will use the global plasma codes BOUT++ (Dudson et al, Comp Phys Sim, 2009) and CENTORI (Knight et al, Comput Phys Commun, 2012) to examine the same effects in full toroidal geometry with a more realistictwo-fluid plasma model. This will enable comparisons to be made with real tokamak experiments.