Probing the plasma pressure limit in tokamaks (plasma strand project)

Supervisors: Professor Kieran Gibson (University of York) and Professor Howard Wilson (University of York)

The fusion power output of a tokamak reactor, such as the experimental ITER facility or future demonstration power plants like STEP, is approximately proportional to the square of the plasma pressure. For a given magnetic field, this pressure is limited by plasma instabilities. One of these instabilities is called the Neoclassical Tearing Mode, or NTM. This results in a bifurcation to a state where filaments of current form in the plasma, which create island structures in the magnetic field. These “magnetic islands” degrade the ability of the tokamak to confine the plasma resulting in a reduced pressure and lower fusion power.

The neoclassical tearing mode typically occurs when two conditions are satisfied: the plasma pressure exceeds a threshold, and a “seed” magnetic island is created in the plasma with a width that is above a critical size. It is important to understand the physics that influences these conditions in order to (a) avoid neoclassical tearing modes and so maximise the plasma pressure and fusion power achievable, and (b) inform the design and operation of NTM control systems. In ITER, for example, a control system based on microwaves has been designed that negates the current filamentation and so shrinks the magnetic island. Knowledge of the critical seed island size is important for optimising this control system.

At University of York we have been developing a new model and simulation code to predict the critical island width. To gain confidence in its predictions, to guide its further development and to reveal further physics insights requires comparisons with experimental measurements on tokamak devices. In this project, we will explore how best to test the code against experiment. Depending on the interests and skills of the student, there is significant flexibility in the project, which could be more towards the simulation end of the spectrum involving further research software development on national-class high performance computers, or more towards the experimental end exploring NTM behaviour on tokamaks such as MAST-U in the UK, DIII-D or NSTX-U in the US and KSTAR in South Korea. In general we anticipate a mixture of simulation and experiment, using both approaches to maximise what we can learn about neoclassical tearing modes in tokamaks and the implications for ITER and STEP.

The technical training you receive will be tuned to your interests, but will likely involve an appropriate mix of computational and experimental plasma physics.

This project will be mainly based in York, but is likely to involve collaborations with experimental teams working on tokamaks both in the UK (MAST-U) and internationally such as in Europe, US and Asia. You will be supported in planning any research visits you make, making use of our extensive virtual meeting and collaboration facilities at York if you are not able to travel in person. This will help you to build an international network of collaborators, which will benefit your PhD research as well as your future employability. Dissemination and discussion are important aspects of research, and you will have opportunities for both UK and international conferences.

This project is offered by University of York. For further information please contact Kieran Gibson (Kieran.gibson@york.ac.uk) or Howard Wilson (howard.wilson@york.ac.uk).

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

Image above: Mirnov coil signals due to a neoclassical tearing mode in the MAST tokamak (J Snape, University of York PhD thesis)