Understanding the L-H Transition in JET Using Gyrokinetic Turbulence Simulations at the Edge in Both L-mode and H-mode – Plasma Strand Project

Supervisors: Moritz Linkmann (University of Edinburgh) and Harry Dudding (UKAEA)

In tokamak experiments a phenomenon can be observed in which the plasma transitions from a low (L) confinement state to a high (H) confinement state as the input heating power is increased beyond a certain threshold. This regime of high confinement, known as H-mode, is characterised by a suppression of turbulent transport and steepening of the pressure profile in a narrow region at the edge of the plasma called the pedestal. Future reactors will almost certainly operate in H-mode however there is currently no theory-based model that can reproduce this L-H transition. This makes development in the understanding of the transition physics a key step towards improving predictions of fusion power plants.

Experimentally, two density branches of the transition have been observed: (a) the low density branch, in which the threshold power decreases with increasing plasma density, and (b) the high density branch, in which the converse is true. This observation suggests something changes in the L-mode edge depending on the local conditions giving rise to multiple avenues of enquiry, such as:

1) Does the nature of the turbulent transport change when going from the low density branch to the high density branch in the L-mode phase?

2) In the resulting H-mode, is there a difference in the transport in a pulse from the low density branch versus one from high density?

3) What signatures of the preceding L-mode transport regime can be identified from the H-mode transport?

4) How are the above influenced by the presence of different isotope masses?

This project will address these questions via high fidelity nonlinear gyrokinetic simulations using the GENE gyrokinetic code, with a high field, high current JET dataset in Deuterium, Tritium and ~50/50 D-T mixtures forming the basis of the simulations. To analyse the turbulent simulation data and place it in the context of the JET experimental results, the student will use data decomposition techniques such as dynamic mode decomposition to gain greater understanding of the spatio-temporal structure of the turbulent fluctuations, elucidating fundamental turbulence phenomena and helping to develop the theory-based understanding of the L-H transition.

The student will be affiliated with the School of Mathematics, entry requirements can be found here.

Plasma strand students are based at University of York for the initial six months of the PhD, for the taught modules. During that first six months students will typically travel to undertake taught modules at all of the Fusion CDT partner universities.

After the taught programme, this project will be mostly based at UKAEA in Culham, Oxfordshire. The student will spend some time in Edinburgh, Scotland.

The student will have opportunities to travel to international conferences and to collaborate with other groups around the world.

This project is being offered by University Of Edinburgh as part of the Fusion CDT Community Studentship scheme. For further information about the project and details of how to apply please contact: Moritz Linkmann (moritz.linkmann@ed.ac.uk) or Harry Dudding (Harry.Dudding@ukaea.uk).

Application deadline for this project: 10th January 2026.