Modelling and Optimising Turbulence in 3D Magnetic Geometries for Enhanced Microstability
This thesis investigates microturbulence in three-dimensional magnetised plasma geometries, with a specific focus on the impact of turbulence to plasma stability. Microturbulence is the small-scale turbulent fluctuations in a plasma caused by instabilities at or near the ion and electron gyroradius scales. Microturbulence remains a critical challenge within the field of plasma physics, and in order to control the effects of turbulence, we first need to accurately model it. This task is particularly complex in non-axisymmetric geometries, where mode-coupling occurs across nonidentical field lines, leading to potential inconsistencies in the common representation of zonal flows. The first part of the thesis focuses on the development and validation of a novel δf-gyrokinetic code, designed to model microturbulence across a full flux-annulus in non-axisymmetric magnetic configurations. This is especially important when considering turbulent dynamics in stellarators and tokamaks with 3D magnetic perturbations, as capturing the coupling of modes across different field lines is necessary to accurately resolve ‘zonal’ modes, that are constant across a flux surface. The code is benchmarked against the current flux-tube implementation of the stella code, along with other existing gyrokinetic codes. Novel results incorporating kinetic electrons are also presented. The second part of the thesis addresses microstability, which poses unique optimisation challenges due to the large number of tunable parameters in magnetic confinement devices. To tackle this problem, an efficient method for calculating the derivative of the linear growth rate with respect to multiple externally-controllable parameters, has been developed using an adjoint method. The theoretical framework is derived in the limit of no field-line coupling, including electromagnetic effects and collisions. Numerical simulations then demonstrate the efficacy of the approach in the electrostatic, collisionless regime.