Turbulent stresses and non-zonal transition in electromagnetic ion-temperature-gradient turbulence

Thesis

The performance of magnetic-confinement fusion devices is strongly influenced by turbulent transport driven by micro-instabilities. While finite plasma beta (β) is often associated with reduced ion-temperature-gradient (ITG) turbulence through electromagnetic stabilisation, recent gyrokinetic studies have identified a transition to high-transport states at sufficiently large beta. Understanding the physical mechanisms underlying this transition is essential for assessing the prospects of high-beta operation in future fusion devices.

This thesis investigates electromagnetic ITG turbulence using a combination of gyrokinetic simulations and reduced fluid models. First, a detailed derivation of the implicit advance scheme employed in the gyrokinetic code stella is presented. The numerical framework is then used to analyse nonlinear electromagnetic turbulence in realistic tokamak equilibria representative of conventional and spherical tokamaks. By deriving expressions for Reynolds, Maxwell and diamagnetic turbulent stresses and the associated energy transfers, it is shown that increasing β modifies the balance of momentum transport responsible for sustaining zonal flows. Near the transition to high-transport states, Maxwell stresses increasingly oppose the Reynolds-stress drive of zonal flows, leading to a reduction of zonal-flow regulation and the emergence of turbulence dominated by nonzonal fluctuations. The scaling of the stress balance with plasma beta is quantified, and convergence studies are performed to establish the robustness of the results.

To gain further physical insight, a reduced fluid model describing electromagnetic ITG turbulence is derived from gyrokinetics by carrying out subsidiary expansion in small electron-to-ion mass-ratio and cold-ion limit. The model captures the essential competition between Reynolds, Maxwell and diamagnetic stresses while remaining sufficiently simple to permit analytical investigation. Linear stability analysis recovers the electrostatic ITG limit, finite-β stabilisation, Alfv´enic dynamics and the high-β interchange regime. Nonlinear simulations of the reduced model reproduce the transition from zonal-flow-dominated turbulence to high-transport states observed in the gyrokinetic calculations. The transition is interpreted as a consequence of a changing the balance between turbulent stresses rather than the onset of a distinct linear instability.

Together, these results provide a unified picture of the non-zonal transition in electromagnetic ITG turbulence. They demonstrate that finite-β effects alter turbulent self organisation through Maxwell and diamagnetic stresses, thereby weakening zonal-flow regulation and enabling enhanced transport. The findings may potentially contribute to the broader understanding of transport limits in magnetically confined fusion plasmas.

Authors: Zhang, Y

• DOI: 10.5287/ora-qydmorpw2
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Fusion plasma