Predicting Pedestal Stability for Alternative High-Confinement Regimes in Future Fusion Pilot Reactors – Plasma Strand Project

Supervisors: Koki Imada and Istvan Cziegler (University of York).

As the world of magnetic fusion research gears toward a working pilot fusion power plant based on the tokamak concept, it is becoming increasingly important to design and validate plasma operational scenarios that can maximise the plasma confinement. One way to achieve this is to operate the plasma in “high-confinement” modes (or H-modes), characterised by the steep radial pressure gradient near the plasma edge, forming a “pedestal” onto which the core pressure profile is elevated. The steeper and higher this edge “pedestal” region is, the better the core confinement will be. However, exceeding pedestal stability limits can trigger detrimental plasma instabilities, such as edge localised modes (ELMs): periodic outbursts of plasma and heat. If large ELMs are triggered in future reactor-scale tokamaks, then the peak particle and thermal fluxes to the vessel walls will be unacceptably high – future devices such as ITER (international collaboration) and STEP (UK) must operate in H-modes without large ELMs.

The stability of the pedestal region depends on various plasma parameters such as the current density and pressure gradient in the pedestal region, the magnetic geometry, and the shape of the plasma cross-section. By carefully tailoring these plasma parameters, it is possible to optimise the pedestal stability and operate in H-modes without large ELMs. These “alternative high confinement” regimes include totally ELM-free quiescent H-modes, so-called “quasi-continuous exhaust” regime with smaller ELMs replacing the detrimental large ones, and “Improved” confinement mode (I-mode) with radially different density and temperature profiles. In order to assess the compatibility of these alternative H-mode scenarios with future reactor-scale machines, it is vital that we understand the underlying physics that governs the pedestal stability and how the ELMs can be suppressed, whilst maintaining good core confinement.

The UK fusion research programme is focusing on the compact spherical tokamaks design for its pilot reactor, STEP (Spherical Tokamak for Energy Production). UK is also home to one of the largest spherical tokamaks currently operating in the world: MAST Upgrade (MAST-U). Because majority of the pedestal stability theory has been based on larger conventionally shaped tokamaks (such as Joint European Torus, JET),  there is an ongoing effort to improve the existing theory to incorporate additional physics arising from the strongly shaped nature of spherical tokamak plasmas, which is crucial in designing no/small-ELM plasma scenarios for STEP, and validating via experiments on MAST-U.

This project will explore the pedestal stability physics of the aforementioned alternative high confinement regimes in strongly shaped plasmas, using the analysis code called ELITE. The project will involve implementing new physics into the ELITE code and benchmarking against existing experimental data for validation. The ultimate goal of he project will be to make predictions for the pedestal stability of no/small-ELM regimes for future MAST-U experiments, as well as informing scenario development for STEP.

The student will benefit from good theoretical and computational plasma background, but over the duration of the project they will gain ample experience in analytic and computational plasma physics, as well as analysis of experimental data. There will also be opportunities for training and professional development in scientific communication.

This project will be mainly based in York, but there will be opportunity to visit UKAEA Culham to participate in MAST-U experiment after 2027.

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: Koki Imada (koki.imada@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.