Understanding Burn-through of Detached States due to Transient Power Pulses in Next-Generation Tokamak Divertors – Plasma Strand Project
Supervisor: Christopher Ridgers (University of York).
Achieving robust, high-performance fusion operation while controlling transient heat and particle loads onto the wall of the device is a major challenge for next-generation tokamaks and future fusion power plants. In advanced tokamak designs, increased power density and compact device size place stringent demands on plasma-facing components, particularly during transient events. Transient heat and particle loading on the divertor (where the heat load is concentrated by the magnetic geometry) from edge-localised modes (ELMs) is of particular relevance to H-mode operation. ELMs pose a serious threat to plasma-facing components, motivating the exploration of alternative regimes such as quasi-continuous exhaust (QCE), in which enhanced but continuous edge transport mitigates large transient loads.
Compact tokamak operation is expected to modify edge stability and turbulence characteristics, potentially favouring access to high-performance H-modes with reduced or absent large ELMs. However, significant uncertainty remains regarding the compatibility of such regimes with detached divertor operation under transient power loading. This uncertainty motivates a detailed investigation of the effect of transient heat and particle pulses on detached divertor states – where impurities radiatively cool the plasma so that it recombines and does not make contact with the divertor wall at all. Addressing this challenge is the central goal of this PhD project.
This project will focus on simulating detachment front dynamics in parameter regimes relevant to next-generation, compact tokamaks using Hermes-3, a state-of-the-art drift-reduced Braginskii fluid model designed for realistic tokamak edge and divertor physics. Building on prior experience with this framework, the research will investigate the burn-through of detachment fronts caused by small, frequent filaments associated with QCE-like behaviour and compare these with larger, more intermittent transient events analogous to ELMs. When energy and particles are injected into the scrape-off layer, there are several mechanisms by which the detachment front may retreat towards the divertor target. Increased thermal transport can raise the temperature at the front, leading to re-ionisation of neutrals—this forms the conventional picture of detachment burn-through.
Recent work, however, indicates that current modelling of this process is deficient in two key aspects. First, plasma and neutral pressure can play an important role, acting in a piston-like manner to displace the neutral cloud. Second, hot electrons can stream collisionlessly into the neutral region on the short timescales associated with transient events, pre-heating the neutral population ahead of the main heat front. Capturing these effects is essential for a realistic description of transient detachment response.
This PhD will deliver a quantitative assessment of detachment burn-through driven by transient power pulses in conditions relevant to future compact tokamak concepts, using newly developed and validated physics models within Hermes-3. By comparing the impact of small, frequent QCE-like filaments with that of larger ELM-like events, the project will establish the conditions under which detached divertor operation can be maintained. The results will inform predictions of divertor heat and particle loads in next-generation devices and clarify the roles of pressure-driven neutral dynamics and kinetic electron transport in transient detachment response. More broadly, this work will contribute to the development of reactor-relevant operating scenarios that minimise divertor damage while sustaining high plasma performance.
The project will be mainly based in York, but there is the opportunity for travel to conferences and collaborations with other groups, particularly at UKAEA. 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: Christopher Ridgers (christopher.ridgers@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: Heat flow accounting for low collisionality of electrons (SNB) and neglecting this (SH). Most scrape-off layer codes neglect this but it makes an important difference to the heat flow – taken from Baker et al. ‘The impact of non-local fluid models on 1D impurity driven detachment in ITER’ 2025 (under review).