Analysing Plasmas in a Tokamak used for Fusion Energy..

Fusion power (the process that fuels stars) has the potential to provide almost unlimited carbon-free electricity and many researchers are confident that it will become usable within the next 40-50 years. However, despite the great progress that has been made since the 1960s, there are still some challenges to overcome before it gets to that stage. These are described in detail in 'Fusion Electricity: A roadmap to the realisation of fusion energy' (Romanelli et al, 2012), a document produced by the EFDA (now EUROfusion). It provides details of a long-term plan for the development of fusion energy from science-based laboratory research up to delivering energy to the electricity grid, stating eight Missions that must be accomplished by 2050. The focus of this research project will relate to Mission 1 of this paper, namely the 'plasma regimes of operation'.

Plasma in a tokamak can become unstable, leading to instabilities and Vertical Displacement Events (VDEs). These are violent disruptions within the plasma and can cause damage to components in the tokamak due to the localised high temperatures and fast-moving particles involved. During a VDE the plasma may also exert large forces on the walls of the vacuum vessel that are transferred to the external structure. It is these effects that the research will focus on and the facility used will be the Mega-Amp Spherical Tokamak Upgrade (MAST-U) at the Culham Centre for Fusion Energy (CCFE). The work will aim to provide insight into how the structure responds to these forces exerted by the plasma instabilities. Additionally, the results may have an impact on the safe operation of the tokamak. There will be two main areas that will be studied to achieve these objectives.

Dynamic monitoring systems are used in MAST-U to observe the behaviour of the vacuum vessel, magnets and structural supports during operation. In particular, they can provide information that can be used to determine the force applied to the various parts during VDEs. This data is emitted from the sensors as signals that may require manipulating before the data may be examined or used for further investigations. Therefore part of the research will be in processing the signal outputs to obtain meaningful results that can be used for additional study. A secondary aim will be to investigate the optimal type and positioning of the sensors.

Once the data is in a practical format, it can be used as an input to a finite element analysis scheme. This will provide solutions to a mathematical model of the tokamak and show the areas where the effect of plasma instabilities are most apparent. Hence the second research area is in analysing this mathematical model, from which the results may be used to guide further investigation in tokamak design and safety.

It is hoped that the methods used in this project can be generalised so that they may be applied to any tokamak scheme. In particular, Romanelli et al make it clear that the ITER project will be a key element of the fusion roadmap, so this research may be a valuable contribution towards the development of that facility.