Transients in the ST Scrape-Off Layer: Reduced Model Development and Validation – Plasma Strand Project
Supervisors: Istvan Cziegler & Christopher Ridgers (University of York) and Jeremy Lore (ORNL).
The energy output from magnetic confinement fusion (MCF) plasmas must be safely removed. One approach is to direct heat flux onto specially designed armoured divertors. Heat from the core plasma crosses the last closed flux surface into the scrape-off layer (SOL), a region of open flux tubes where it is rapidly transported to the divertor. In most MCF power-plant designs this heat is mitigated by cooling the plasma until it recombines, forming a radiative buffer between the hot core and the divertor surface, a phenomenon known as detachment.
Modelling the response of this detached plasma to slow power transients remains a significant challenge, particularly for MAST-U, which incorporates advanced divertor configurations. Events such as L-to-H transitions, changes in plasma current, gradual equilibrium shifts, or unexpected variations in core parameters (including core radiation fluctuations) can strongly affect detachment. Understanding how the detachment front evolves, and potentially re-attaches, under such transients, along with the associated timescales, is essential for designing stable operational regimes and informing divertor control strategies.
Although established tools such as SOLPS are widely used, comparatively little effort has focused on time-dependent, reduced models capable of efficiently capturing transient divertor dynamics. This project addresses this gap by developing time-dependent simplified models of electron and neutral kinetics within the BOUT++ framework, with particular emphasis on the Hermes-3 code. Hermes-3 is well suited to high-temporal-resolution transient studies, enabling efficient parameter scans, sensitivity analyses, and the creation of datasets suitable for machine-learning applications.
A central aim is to benchmark the reduced Hermes-3 models against high-fidelity tools such as SOLPS and, where feasible, hybrid-kinetic codes like REMKiT. These comparisons will identify where reduced models reliably reproduce the behaviour of high-fidelity simulations, highlight their limitations, and guide further improvements. The computational work will be closely connected to experimental data from MAST-U. The student will analyse divertor measurements, such as spectroscopy, Thomson scattering, infrared thermography, bolometry, and detachment-front tracking, to validate and constrain the models. By generating synthetic diagnostics and comparing them with MAST-U observations, the predictive accuracy of the models under realistic conditions will be assessed.
As part of the Fusion CDT, the student will receive extensive training in both theoretical and computational aspects of fusion research. They will develop high-performance-computing skills, including use of national and international supercomputing facilities, parallel programming, performance optimisation, and advanced data processing. In modelling and simulation, the student will focus on developing time-dependent reduced models in BOUT++/Hermes-3, benchmarking against SOLPS and hybrid-kinetic codes, applying verification and validation techniques, and producing synthetic diagnostics with uncertainty analysis. The student will also gain experience with experimental work, including analysis of MAST-U divertor diagnostics and integration of modelling with experiment. Professional and transferable skills will be developed through scientific communication, project management, collaboration, participation in workshops and summer schools, and training in computational, mathematical, and data-science methods.
This project is a partnership between the University of York and Oak Ridge National Laboratory, providing access to international expertise in edge modelling, plasma-neutral interactions, and high-performance computing. The collaboration may involve joint projects and research visits, giving the student exposure to modelling approaches and supercomputing resources used in the US fusion programme.
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: Hermes-3 simulations of a transient power event – the blue line includes the effect of momentum transfer, which can be seen to have a large effect. The physical processes affecting detachment front motion are not yet well understood. Taken from Khan et al, ‘The role of momentum transfer in the detachment front response to power transients for reactor scale tokamaks’. 2025 (under review).