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Understanding Helium Exhaust for Future Fusion Reactors using MAST-U and SOLPS-ITER – Plasma Strand Project

Supervisors: Christopher Ridgers (University of York) and Ryoko Osawa (UKAEA).

As we move toward realizing commercial fusion power, one of the key engineering and physics challenges is how to remove helium “ash” efficiently. In fusion reactions using deuterium and tritium (D-T), helium is produced as a byproduct. While it’s harmless in itself, if not removed effectively, this helium can accumulate and dilute the fuel, reducing the reactor’s performance.

At the same time, efficient helium removal (or helium exhaust) isn’t straightforward. In the divertor – the part of the reactor designed to handle waste heat and particles – maintaining a buildup of neutral particles is actually helpful for protecting the machine. So there’s a trade-off: removing helium too aggressively can disrupt the conditions needed for safe and efficient divertor operation. Balancing these opposing needs is one of the critical challenges for the design of future reactors like STEP (Spherical Tokamak for Energy Production).

Despite this, our understanding of how helium behaves and is transported in the Scrape-Off Layer (SOL) and divertor region is still limited. Recent experiments on international machines like ASDEX Upgrade and DIII-D have shown clear discrepancies between measurements and model predictions – highlighting the need for better models and more data to validate them.

This PhD project aims to address this problem by combining experiments on MAST-U (Mega Amp Spherical Tokamak Upgrade) with advanced plasma simulations using SOLPS-ITER, a state-of-the-art edge plasma code. The project will explore two key questions:

  1. How well can current models reproduce the transport and distribution of helium in MAST-U?
  2. What changes to the plasma configuration can help reduce helium buildup in the core?

The work will begin with steady-state plasma discharges, avoiding transient events like Edge Localised Modes (ELMs), which complicate analysis and are not suitable for future reactors. The student may also develop synthetic diagnostics – simulated measurement tools – to make direct comparisons between experimental data and simulation results. Once this comparison is understood, the focus will shift to exploring how changes in plasma shaping or divertor geometry can improve helium exhaust.

This project is highly relevant to UKAEA’s MAST-U and STEP programmes, both of which are critical to the UK’s fusion roadmap. It also supports the goals of the University of York’s Fusion CDT (Centre for Doctoral Training), which aims to develop the next generation of fusion scientists and engineers.

As a PhD student, you will:

  • Be based at the UK Atomic Energy Authority (UKAEA) – the UK’s national fusion research lab.
  • Work at the cutting edge of experimental plasma physics and computational modelling.
  • Contribute to the ongoing design of STEP, a potential future power plant.
  • Gain skills in plasma diagnostics, simulation, and reactor-relevant plasma operation.

This is an ideal opportunity for a motivated physics, engineering, or applied mathematics graduate who is excited by the challenge of fusion energy and wants to work on a problem with both deep scientific interest and real-world impact.

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, for the remainder of the PhD this project will be based at UKAEA in Culham, Oxfordshire.

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: SOLPS simulations for STEP divertor design – taken from Henderson et al., Nucl. Fusion 65 (2025) 016033.