The influence of magnetic geometry on the plasma edge region of future fusion reactors

Je-S summary: Developing a portfolio of energy producing solutions is imperative to advance the economy, keep society functioning and to fight the advance of climate change. Commercial fusion will play an important role as part of that portfolio when technical challenges are overcome, having the ability to provide a low-carbon, essentially limitless, steady source of energy on a large scale, with a small land footprint.
Arguably the biggest challenge to the magnetic fusion energy concept is how to safely channel the reactor heat exhaust to the surrounding material surfaces. The fusion reactions occur between ions in the hot fusion core that are at a temperature ~ 100 million oC. This provides enough energy for the positively charged nuclei to overcome their electrical repulsion and come close enough for the attractive nuclear force to take over. The two ions then bind to form a new nucleus with less mass than the original two ions, releasing energy. The hot plasma is contained within a magnetic 'bottle', called a tokamak, which has the shape of a torus. Exhaust energy and particles, which leak out of the magnetic bottle in steady state, are diverted away along magnetic field lines to remote material surfaces (termed 'divertor targets') designed to handle the resulting high power densities. The excellent confinement of energy within the magnetic bottle necessary for fusion energy production results in a narrow channel of exhaust power flowing to the divertor targets which, for a tokamak of radius ~ 5m, has a channel thickness of order 1 mm. The resulting heat flux flowing along the magnetic field to the divertor target likely approaching 25GigaWatts/m2. This is about 500x times the heat flux of an arc welder and 2500x what the engineering limits of steady state heat transfer to a solid material allows (10MegaWatts/m2).
We employ several methods to reduce this heat flow to surfaces to below engineering limits: a) The simplest is to arrange the angle of the target to the heat flux to be small, spreading the divertor heat over a larger area, reducing the peak heat flux by x20; & b) More significantly, we encourage light (power) to be emitted from the plasma. We also utilise other atomic processes to remove energy, momentum and even particles from the plasma in a process we call 'detachment'. Detachment has been the main process to reduce the heat flux to the target, but more reduction is needed for a viable reactor-scale device.
The goal of this research project is to evaluate how modifications of the magnetic fields and geometry in the divertor target ('alternative divertor configurations') region can further enhance the power removal properties of the plasma & reduce the heat fluxes reaching divertor surfaces below the engineering limit. In the research proposed we will: a) test our model predictions that alternative divertor configurations remove more heat from the plasma & better control the detachment processes using data from existing and new diagnostics we develop; and b) study both the dynamics of how the plasma is cooled through the processes mentioned and the sensitivity of the detachment process to external controls.
This project will promote the UK into a world-leading role in the area of fusion reactor divertor physics research through development of key knowledge and research capabilities within the UK. Indeed, we will contribute unique results to the upcoming EU decision of what the appropriate divertor solution is for commercial reactors, reducing the time to a demonstration fusion power plant. The proposed work will also accelerate fusion research at the £50M upgrade to the MAST tokamak at Culham, where the first (worldwide) embodiment of the so-called 'super-x' alternative divertor topology will occur. By the UK playing a key role in achieving fusion the country will benefit economically from commercial applications as well as having an essentially limitless, steady source of clean energy into the future.

Je-S Impact Summary: Developing a portfolio of energy producing solutions is imperative to advance the economy, keep society functioning and to fight the advance of climate change. Commercial fusion will play an important role as part of that portfolio when technical challenges are overcome, having the ability to provide a steady source of energy on a large scale, with a small land footprint. Fusion energy also provides a low-carbon, sustainable, essentially limitless, energy source.
The research in this proposal will accelerate the progress towards commercial fusion energy production, benefitting the UK, and the world. Our proposed work, based on our model predictions, will compare several potential enhancements, or alternative solutions to the tokamak fusion reactor device power exhaust capability, called a 'divertor'. Those enhancements will both increase reactor power exhaust capability, allowing smaller and more cost-effective reactors, and better control of heat exhaust physical processes, imperative for ITER (the penultimate step to a reactor being built by the EU and worldwide partners in France) and DEMO (the proposed demonstration fusion nuclear reactor expected to be operational in the second half of this century). The EU roadmap to fusion energy* states that because heat loads predicted for the DEMO divertor are so high 'an aggressive programme on alternative solutions for the divertor is necessary'.
This project will promote the UK to a central, if not leading role in the key area of fusion reactor divertor physics research through: (1) development of key knowledge and research capabilities within the UK, and (2) providing an accelerated, or earlier assessment of the various alternative divertor solutions, thus providing critical knowledge to the upcoming EU, and worldwide decision of what the appropriate DEMO divertor solution is. In addition we will have more confidence in a fusion reactor concept as well as be ready earlier to build DEMO. The proposed work will also accelerate fusion research at the £50M upgrade to the MAST tokamak at Culham, of which a central focus is the first (worldwide) true embodiment of probably the most-promising divertor topology alternative, the super-x divertor. By the UK playing a key role in achieving fusion the country will benefit economically from commercial applications as well as a steady source of energy into the future..
The plasma and atomic physics associated with power exhaust has much in common with low-temperature plasmas which have a range of applications. As well as displays and lighting, there are other applications of these plasmas, such as the erosion, functional modification and coating of surfaces. Applications span a wide range, from human tissue to mirror surfaces, as well as ways to create new surface materials that are advantageous in terms enhancing hydrogen production (for fuel and storage) and catalysis of gases. Beyond surfaces the physics of heat exhaust has similar physics to that associated with spacecraft plasma thrusters that utilise embedded magnetic fields.
There will be three post-doctoral Research Associates involved in this research. PDRA1 & PDRA2 will have an experimental emphasis, whilst PDRA3 will be focussed on modelling. This work will provide a deep education in divertor and plasma physics in the effort to understand the differences amongst the three divertor configurations studied. They will have a unique training applicable to fusion research and, more generally, in the use of plasmas for commercial applications mentioned above. Three PhD students will be involved in the programme (funded through the EPSRC CDT in Fusion Science & Technology led by York). Similar to the PDRAs they will also be provided with unique training that will make them well-equiped for careers across fusion and commercial plasma applications.
* https://www.euro-fusion.org/wpcms/wpcontent/uploads/2013/01/JG12.356-web.pdf