Modelling Impurity Transport in Turbulent Plasmas and Extensions Toward Realistic Conditions – Plasma Strand Project

Supervisors: Istvan Cziegler & Christopher Ridgers (University of York) and Jeremy Lore (ORNL).

Transport across the confining magnetic field, dominated by turbulence-driven fluxes, remains a major challenge for magnetic confinement fusion. The plasma edge and scrape-off layer (SOL), located around the last closed flux surface, are particularly complex due to strong nonlinear dynamics and the presence of multiple species, including fuel ions, helium ash, injected impurities, and eroded wall materials. Their interaction with turbulent structures leads to complex, often non-diffusive transport. Coherent features such as filaments, vortices, and shear flows are known to influence transport and can form barriers, yet their role in regulating impurity motion in the edge–SOL remains poorly understood.

A key difficulty in impurity-transport modelling is the lack of a comprehensive fluid description. Impurities often violate standard fluid assumptions, particularly at low concentration or with charge-to-mass ratios distinct from background ions. Traditional models treat impurities as passive tracers, but more realistic approaches must include finite inertia, charge-to-mass effects, and non-thermal dynamics. A practical strategy is to begin in the small-concentration limit, treating impurities as particles evolving in turbulent fields, before progressing toward fully self-consistent models with impurity feedback on the plasma.

At this initial stage, the 2D Hasegawa–Wakatani (HW) model provides a minimal yet physically grounded description of drift-wave turbulence, capturing instability-driven fluctuations, energy transfer, and shear-flow dynamics. Because rapid parallel transport makes much of the SOL effectively two-dimensional and nearly isothermal, HW-type models offer a credible first approximation to edge–SOL turbulence, particularly near boundaries or partially detached divertor conditions.

This project aims to develop time-dependent reduced models of turbulence and impurity dynamics within the BOUT++ framework, using the HW model as a foundation and extending toward edge–SOL conditions relevant to MAST-U. Early work will treat impurities as test particles to study cross-field transport, interactions with coherent structures, and non-diffusive behaviour. Later stages will introduce self-consistent effects such as impurity modification of charge balance, potentials, and local turbulence.

Implementation will involve selecting appropriate particle-following schemes (e.g. Boris, Vay, or gyro-averaged methods), leveraging existing or developing BOUT++ particle modules, and incorporating simplified collisional, charge-exchange, and charge-state evolution models where feasible. These developments will enable future extensions to 3D geometry, sheath boundary conditions, and neutral-plasma interactions, facilitating comparison with MAST-U diagnostics.

By the end of the project, the student will have developed and validated reduced impurity-transport models; identified how turbulence, coherent structures, and shear flows regulate impurity motion; benchmarked models against BOUT++ simulations and experimental signatures; and clarified the regimes where reduced models succeed or fail. The work supports the broader goal of understanding multi-species, turbulence-driven transport in fusion plasmas and contributes directly to predictive capability for impurity control in future devices.

The project is embedded within the Fusion CDT, providing training in fusion theory, computation, and experiment, alongside high-performance computing, numerical modelling, data analysis, and collaborative research skills.

This project is a partnership between the University of York and Oak Ridge National Laboratory, providing access to international expertise in edge and SOL modelling, plasma–neutral interactions, and advanced computing. Opportunities for collaboration and research visits will give the student exposure to state-of-the-art turbulence modelling, impurity physics, and supercomputing environments used across the UK and US fusion programmes.

During the first six months of the PhD students will typically travel to undertake taught modules at all of the Fusion CDT partner universities.

This project is offered by University of York. For further information please contact: Istvan Cziegler (Istvan.cziegler@york.ac.uk).

This project may be compatible with part time study, please contact the project supervisors if you are interested in exploring this.

For details on how to apply, please visit: Apply

Image above: Hasegawa–Wakatani simulation of edge turbulence with sample impurity particles stuck in vortices as zonal flow patterns form.