Performance-limiting Plasma Instabilities in ITER – the Neoclassical Tearing Mode (Plasma Strand Project)

In a tokamak, magnetic field lines lie within a set of toroidal surfaces that are nested inside the plasma like a set of Russian dolls. These surfaces are called flux surfaces. Heat and particles flow readily along magnetic field lines, but only diffuse slowly across them. The result is that the plasma pressure is constant on the flux surfaces, but a strong pressure gradient is maintained across them. This, in turn, allows high pressure to be achieved in the plasma core and, in a next step tokamak like ITER, this will provide the required fusion power – fusion power density is approximately proportional to the square of the plasma pressure.

As the pressure is increased in an effort to maximise the fusion power, a number of plasma instabilities can form. A particular kind of instability is called a neoclassical tearing mode. This results in a filamentation of the current density that flows inside the plasma, leading to a set of so-called magnetic islands. These magnetic islands cause a reduction of core plasma pressure and, on ITER, a consequent drop in fusion power. Avoiding or controlling neoclassical tearing modes is crucial in order to maximise the fusion power in ITER.

The neoclassical tearing mode mechanism acts to amplify a “seed” magnetic island if its width exceeds a certain threshold, while below this threshold any seed magnetic island is observed to be healed by the plasma. At York, we have developed a new theory and associated computer code to understand the mechanism by which islands are healed [K. Imada, et al Phys Rev Letts 121 (2018) 175001]. The aim of this PhD project will be to build on this new work to quantify how to avoid or control neoclassical tearing modes in ITER. The research will be predominantly theoretical, but will seek to compare with experiments on the UK’s MAST-U tokamak, as well as other tokamaks around the world (JET (UK), DIII-D (US), NSTX-Upgrade (US), AUG (Germany) and KSTAR (S Korea).

You will be mainly based in York, but there will be many opportunities to travel to Culham and indeed internationally. You will be responsible for extending and exploiting an existing computer code, written in f90, with simulations to be performed on large supercomputers, such as ARCHER. Some coding experience would be an advantage (not essential), but you would be given a lot of support and training to learn how to work with high performance computers. You will also learn new advanced theoretical skills. In working with experimentalists, you will get a good understanding of experimental techniques and the challenges of comparing theory and experiment. The results are expected to be of international interest, and you will be required to lead journal publications and give presentations at conferences (supported by your supervisor).

This project is offered by University of York and will be supervised by Prof Howard Wilson (howard.wilson@york.ac.uk). For further information please contact Howard Wilson. (howard.wilson@york.ac.uk)