Plasma turbulence in 3D magnetic fields

Small scale instabilities (microinstabilities) are excited in magnetized plasma by large scale inhomogeneities in the plasma density and temperature. The associated turbulent transport of particles and energy limits the confinement of tokamak plasmas, i.e., plasmas immersed in axisymmetric-and thus two-dimensional-magnetic fields. Consequently, much effort has gone into understanding the properties of turbulence and turbulent transport in tokamaks. However, perfect axisymmetry of the confining magnetic field is marred by various phenomena. These include design constraints (e.g., the discrete placement of toroidal field coils leads to ripple in the toroidal magnetic field), large-scale plasma instabilities, and resonant magnetic perturbations that are applied to suppress undesirable instabilities localised to the edge of the plasma (called edge localised modes or ELMs). Despite the prevalence of such 3D modifications, there has been little work done on turbulence in three-dimensional magnetic fields. The main aim of this project is to understand how breaking axisymmetry affects plasma microstability and turbulent transport.

This would involve a combination of code and algorithmic development, analytic theory, and mathematical modelling. In particular, the student would aim to: extend the 3D, flux tube gyrokinetic code stella to simulate an annular region surrounding a magnetic flux surface (making it a `full flux surface' code), which is needed to accurately capture 'zonal' modes that span multiple magnetic field lines within a flux surface; develop a novel adjoint method for efficiently optimising the design of the confining magnetic field to improve microstability; extend earlier analytic calculations of how nonaxisymmetric perturbations modify microstability and transport; and use a combination of analytical models and numerically-constructed 3D equilibria to model MAST-relevant tokamak plasmas. At the least we aim to develop a qualitative understanding of how microstability and transport are modified in 3D fields: Ideally, we would also be able to take this understanding and apply it to phenomena such as ELMs suppressed by resonant magnetic perturbations and/or transport in the presence of long-lived magnetohydrodynamic (MHD) modes. Finally, we expect that what we learn from this project will inform studies of transport and microstability in stellarators, which use the additional freedom associated with the 3D nature of the confining field to optimize macroscopic stability and confinement.

This research has the potential to change the direction of stellarator design (as well as plasma shaping in tokamaks) and to inform the design and use of resonant magnetic perturbations to stabilise ELMs in tokamaks. Each of these outcomes would have a significant impact on the pathway to designing magnetic confinement fusion reactors.

This project falls within the EPSRC research areas of Plasma and Lasers and Numerical Analysis. It is to be carried out in collaboration with researchers at the Culham Centre for Fusion Energy, in particular with Dr. Sarah Newton who will serve as a co-supervisor for Georgia Acton.