The role of neutral baffling in current and future tokamaks – plasma strand project

Supervisors: David Moulton (UKAEA) – main supervisor & Christopher Ridgers (University of York).

Dealing with the extreme heat at the edge of a magnetically confined fusion (MCF) device is one of the major outstanding problems for a reactor-scale fusion device.  The core of such a plasma is at around 100 million degrees, yet the edge of the device must be kept at manageable heat loads for material surfaces.  In most MCF devices the heat is diverted along open magnetic field lines at the edge of the device onto a heat-resistant target plate in the so-called divertor region. One solution to reduce the heat load on the target is to puff an impurity into the divertor to radiate the energy away and create a cloud of neutrals – detaching the plasma from the target.  Rectors must operate in this detached regime if the diverter target is to survive and so facilitating detachment is crucial.

The baffling of hydrogenic neutrals, trapping them in the divertor region, appears to facilitate detachment in current tokamaks. However, baffles add cost and complexity to the divertor design. They also restrict the available magnetic shapes for scenario development (including ramping up and down and strike point sweeping during the active phase). This makes it important to understand how the impact of baffles scales to a reactor such as STEP. In a reactor, power dissipation is likely to come primarily from seeded impurities, making the confinement of neutral impurities more important than on current machines. In addition, reactors may already have sufficient neutral trapping without baffles, due to their shorter mean free paths. This project aims to improve our understanding of neutral baffling in current and future machines, through a combination of experiment (MAST-U and possibly TCV) and interpretative and predictive simulation (using the SOLPS-ITER and HERMES-3 codes; a useful side effect will be a comparison between the HERMES-3 and SOLPS- ITER neutral models). After benchmarking MAST-U simulations against experiment (including comparisons of neutral pressure measurements), the candidate will conduct code experiments at reactor-relevant parallel heat fluxes, with and without impurities, and with and without baffles in the simulations. Importantly, within this project, we will also test the sensitivity to different neutral rates used in the code, for example the molecular charge exchange rate. If time allows, the candidate will repeat these scans in Conventional and Super-X geometries, in order to discern the relative importance of baffling and magnetic geometry, in current machines and reactors.

This project will be primarily based at UKAEA in Culham. It will give the candidate the opportunity to experience both academia and a national laboratory, helping them to develop skills working in diverse environments and make a wide range of contacts. The project will involve the development of programming and high performance computing skills highly useful in the UK’s knowledge-based economy. It will also involve close interaction with experiments facilitating a practical understanding of cutting-edge scientific instruments and devices. The project is suitable for both part-time and flexible working arrangements.

The project will mainly be based at UKAEA but there is opportunity to travel for conferences and to collaborate with other groups.

This project is offered by University of York. For further information please contact: Christopher Ridgers –

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