Stability of the Plasma-Wall Boundary in Fusion Devices – Plasma Strand Project

Supervisors: Alessandro Geraldini and Jonathan Graves (University of York).

The plasma-wall boundary conditions in magnetic fusion devices always assume that the boundary layer that forms next to the wall, the magnetised plasma sheath, is stable. This assumption is a reasonable one in many plasmas of interest, but fusion devices are peculiar in that the magnetic field impinges on the device walls at an oblique but very shallow angle (2-5 degrees). This project will investigate the stability of the plasma-wall boundary layer in a fusion device and its impact on plasma-wall boundary conditions, which are a key element in the prediction of particle and energy transport close to the device walls.

While it is known that instabilities are present next to a wall when the magnetic field is exactly parallel to the wall [1], the literature is lacking detailed studies of stability at grazing magnetic incidence. In this scenario, there are two possible channels of ion transport to the wall: by parallel streaming along the magnetic field line, and by drifts across the magnetic field caused by density and electrostatic-potential gradients tangential to the wall. For sufficiently small magnetic field angle, the ion transport by parallel streaming no longer dominates, as the ion transport by drifts is comparable. Our recent studies have shown that this situation may be inconsistent with a steady state in the plasma-wall boundary layer [2].

The project will involve building upon the generalised gyrokinetic equations describing ion transport in the boundary layer [3] to allow for time dependence. The generalised gyrokinetic equations extend the conventional ones by allowing the gyro-orbits to be distorted from approximately circular to non-circular, which is a key physical process that occurs in the boundary layer. Only the slowest timescale, constrained by the transport of ion gyro-orbits across the layer, will need to be considered, allowing for the development of a set of gyrokinetic equations from which insight can be gained onto the physical mechanisms at play and whose solution is vastly less computationally expensive than a conventional particle-in-cell code. The project will therefore involve both analytical and numerical work.

Under what conditions will the boundary layer transition from being stable to unstable? What is the nature of the instability? If it can occur in fusion devices, how should the boundary conditions be modified? These are some of the questions which this project aims to answer.

[1] K. Theilhaber and C. K. Birdsall, “Kelvin–Helmholtz vortex formation and particle transport in a cross-field plasma sheath. I. Transient behavior”,  Physics of Fluids B: Plasma Physics 1, 2244 (1989); https://doi.org/10.1063/1.859041

[2] A Geraldini et al, “Sheath constraints on turbulent magnetised plasmas”, Plasma Phys. Control. Fusion 66 105021 (2024); https://doi.org/10.1088/1361-6587/ad705a

[3] A Geraldini et al, “Gyrokinetic treatment of a grazing angle magnetic presheath”, Plasma Phys. Control. Fusion 59, 025015 (2017); https://doi.org/10.1088/1361-6587/59/2/025015

The project will be mainly based in York, but there is the opportunity for travel to conferences and collaborations with other groups, in particular to EPFL.

During the first six months of the PhD plasma strand 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: Jonathan Graves (j.graves@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