Tokamak Energy Ltd is a private company targeting the delivery of fusion as a clean and safe energy source by 2030. The company aims to do this by combining spherical tokamaks, which are a type of magnetic confinement fusion device, with high temperature superconducting (HTS) magnets, which can deliver very strong magnetic fields in compact devices. The company believes that HTS spherical tokamaks are the key route to delivering commercial fusion energy on a rapid timescale. This project aims to address two key challenges in the field of HTS magnet technology, in order to accelerate the development of HTS magnets for fusion energy and other applications. The first challenge is to develop a technical and strategic approach towards the characterisation and quality assurance (QA) of HTS conductors, then implement this on several hundred kilometres of conductor procured over a period of several years. The key difficulty here is that the current capacity of rare-earth barium copper oxide (REBCO) HTS conductors is extremely large and has a very complex dependence on temperature, magnetic field strength, field direction and the crystal's nanostructure. Measurement of conductor performance under the end-use conditions in fusion magnets is extremely challenging due to the high magnetic fields and currents involved. Therefore, complete characterisation cannot be carried out routinely despite magnet designs relying crucially on their knowledge. This project will establish the necessary performance indicators (balancing cost, risk and depth of information), develop the methods required to measure them, and implement this on the real conductor as it arrives. The second challenge is the development of dismantlable coil structures for HTS fusion magnets. HTS magnets can be operated at relatively high temperatures (>~20 K) at which substantial heat loads from joints between conductors can be accommodated by cooling systems. Unlike conventional low temperature superconductors (LTS), HTS conductors operated at high temperatures are extremely thermally stable and can therefore tolerate substantial temperature variations of several degrees Kelvin around their structures. This enables dismantlable coil structures to be considered, in which the turns of the magnet can be connected and disconnected from one another during assembly and disassembly. This is an extremely attractive design option for tokamak magnets, where it is advantageous for some coils (e.g. poloidal field coils (PFs) ) to be threaded inside other coil sets (e.g. the toroidal field coils (TFs)). The wider assembly process for tokamaks is also greatly simplified if the coils are dismantlable, for example the assembly of vacuum chambers and neutron shields. Development of dismantlable coils is a multifaceted problem involving development of novel low resistance jointing methods, practical implementation methods in a tokamak assembly hall environment, and design of the wider magnet system to accommodate the joints (including insulation methods and magnet operating principles).

There has been no greater existential threat to humanity to date from anthropogenic climate change as a result of CO2 emissions. The effects are already apparent in terms of more extreme weather, loss of polar ice and rising sea levels. Power generation contributes to much of the CO2 emissions from fossil fuel burning as the worldwide demand for power continues to outstrip supply. Renewable energy (wind, solar, hydro) is limited by weather dependency, with associated issues such as energy storage and land use. Nuclear fusion has a significant role in decarbonizing global power generation but radiation shielding is a limiting factor. Creating miniature Suns is one part, but materials must exist that can withstand fusion conditions for practical fusion reactors. W-based alloys and other refractory metals are current solutions in fusion reactors, but the engineering requirements for power-generating fusion reactors exceed those in current materials. The goal of this fellowship will demonstrate the feasibility of radiation shielding based on the Cemented Tungsten Carbides and Reactive Sintered Borides (cWC-RSB) concept in Compact Spherical Tokamaks (cSTs). cWC-RSBs can bridge the gap between current materials and the engineering requirements for a power-generating cST. cWCs-RSBs have excellent radiation absorption properties by combining heavy (W) and light elements (C, B) with the strength and toughness by combining WC with a ductile metal binder. However, cWCs have never been used in nuclear reactors to date since the use of Co (and Ni) metal as a binder alloy prevented the use of cWCs as radiation due to Co and Ni being activation hazards. In 2014, I discovered that non-activating FeCr alloys are suitable as cWC binder alloys, with RSB development following on investigating boron additions in cWCs. Combined cWC-RSB shields have greater radiation attenuation overall, compared to cWCs alone. The first objective evaluates the thermo-mechanical properties and the safety case for shielding candidates, including high-temperature oxidization and thermal shock to in terms of worst-case scenarios, such as exposure of hot shielding to air. Experimental data on Si-coated cWCs showed that Si-coating retarded oxidization rate by 4 orders of magnitude relative to tungsten in the temperature range 900C-1200C. I will evaluate the properties of cWC-RSBs over cryogenic to failure (> 1200C) temperatures predicted for power-generating fusion reactors. While considerable data on the thermo-mechanical properties exist for cWCs since the 1930s, there is little on RSBs, given their novelty and it is crucial that thermo-mechanical properties of RSBs are well-known prior to industrialization. The novelty of RSBs means that very little is known about their chemistry and routes to fabrication. Current processing methods are not fully optimized for dense, crack-free RSBs. The second objective aims to fill these gaps using the calculation of phase diagram method (CALPHAD) for predicting the most suitable compositions and design of experiment (DoE) methodology for the most efficient processing trials. This research demonstrates how new solutions can be derived from existing materials and techniques when a critical gap in current solutions is apparent. Recent simulations of the neutron and gamma attenuation of WC- and RSB-based shielding concepts show considerable promise. However, there is little data on the radiation response of cWCs and none on RSBs to date. For this third objective, I intend to build on current research using simulated cSTs to inform radiation experiments and experimental work simulating the range of conditions inside a cST, including ion bombardment, charged particles, and secondary radiation. Data from cWC-RSB shields in a simulated fusion reactor alongside demonstrated oxidization resistance indicates that cWC-RSB materials exceed current radiation shielding candidates in terms of radiation attenuation and safety.

Nuclear fusion - the joining together of atomic nuclei of light elements such as hydrogen to form larger nuclei - is the process by which vast amounts of energy is produced in stars like our sun. If it can be harnessed on Earth it has the potential deliver a nearly unlimited and safe source of energy which does not produce the environmentally damaging CO2 emissions that are released by burning traditional fossil fuels. However, for nuclear fusion to occur, extremely high temperatures and pressures are required because positively charged atomic nuclei within a plasma have to collide with each other with sufficient energy to overcome the immensely strong electrostatic repulsion forces. To achieve nuclear fusion in a machine on Earth, extraordinarily high temperatures of around 150 million degrees Celsius are needed, about 10 times higher than the temperature of the sun's core. This precludes the use of traditional materials to confine the plasma, and in the most common type of fusion reactor called a tokamak, strong magnetic fields are used instead. Since the power density of a particular geometry of tokamak scales with the strength of the magnetic field to the power of four, there is a huge benefit to using higher field magnets for plasma confinement. High temperature superconductors - materials that can conduct electricity without any resistance - are an enabling technology for a new generation of compact nuclear fusion reactors that are widely believed will open the door to commercialisation of fusion for energy generation. This is because state-of-the-art high temperature superconducting tapes can carry extremely high electrical currents, even when subjected to enormous magnetic fields that completely destroy superconductivity in the best low temperature superconductors. However, although high temperature superconducting materials with fantastic properties are now available in lengths up to about 1 km in the form of flexible tapes known as coated conductors, the materials are incredibly complex and sensitive to damage, making their practical deployment in magnets for fusion devices a major challenge. This programme of research involves using a unique combination of advanced materials characterisation and modelling techniques to determine how high temperature superconductors will degrade in the harsh environment of a fusion reactor where they will be continually bombarded by high energy neutrons. The focus will be on understanding the underlying damage and recovery mechanisms in these complex functional ceramics under the most realistic conditions possible. Since in operation the superconductors will be irradiated by neutrons whilst in their superconducting state at cryogenic temperatures, innovative in situ experiments will be performed to understand the differences between room temperature and low temperature radiation damage. The experimental programme will be supported by first principles modelling of pristine and defect structures in the superconducting compounds, and the outcomes will be used to validate larger scale simulations of radiation damage as well as providing key data on degradation to feed into materials selection and magnet design decisions for the next generation of fusion magnets. The advanced characterisation methodologies developed in this fellowship will also be applied to understanding radiation damage in a wider range of fusion relevant materials.

The process powering the sun can be harnessed as clean and safe fusion energy. Progress in fusion could be accelerated by shrinking the size and cost of reactors and the UK Government has recently announced £220 million to develop such smaller reactors. However, for them to operate continuously for several decades, certain parts of the reactor must be shielded from high energy particles. With currently available shielding materials, these parts will begin to degrade within a matter of weeks or months. My programme of work will develop more efficient shielding materials so that these reactors can operate on a continual basis. Conventional shields use heavy atoms, which reflect the lighter particles; similar to how a ping-pong ball might bounce back off a snooker ball. My research is based on a hybrid approach, combining heavier elements with lighter ones, which instead absorb and dissipate the particle's energy; think now of similarly weighted balls colliding, like a break in snooker. The approach has been proven in theory, but I must now turn this into reality by fabricating and testing real engineering materials. In doing so I will work closely with the UK's leading fusion engineering company, Tokamak Energy, and the UK Atomic Energy Authority, both of whom seek to build energy-producing reactors within the next 10-20 years. My first aim is to fabricate these materials. Because they are very hard and do not melt easily, I will use similar methods to the way other hard materials are made, such as those within a household drill-bit. These are made by compressing powders together at high temperature so that the powders fuse to form a solid material. I will test the properties of the materials like their strength. As part of this I will seek to understand how the geometrical arrangement of the atoms within the material - the so-called "microstructure" - affects these properties. The second aim will be to understand how these materials degrade in the environment of the fusion reactor. They will be subjected to extreme heating, which in some areas of the reactor is similar to what is experienced in a rocket engine. I will test how the material's mechanical strength degrades at these temperatures, just like steel is softened in a blacksmith's furnace to become malleable. At the same time, the materials will also be bombarded by high energy particles in the reactor. This tends to jumble-up the arrangement of the atoms, which can make the materials more brittle; in the same way that when you bend a paperclip back-and forth, it eventually snaps. To test this, I will use specialist particle beam facilities to simulate the damage process. Because the damage only occurs on a small scale (about a tenth the thickness of a human hair) I will use very high-power microscopes to observe the jumbling-up process. I will also perform small-scale mechanical tests on the damaged areas to understand how the jumbling-up effects strength. To interpret these tests, I will work with experts in computer modelling, who can simulate individual "atomic jumps" to work out which sorts of jumps are responsible for the damage. The final aim of the fellowship is to optimise the material's atomic arrangement to improve its damage tolerance. To achieve this, I will engineer the material's building blocks by firstly adding a cement-like layer between blocks, and secondly by flattening the blocks like pancakes. Such engineering is found in nature, where sea-snail shells are built from thinly stacked layers of relatively brittle chalk-like ceramics, with a gluey substance in between. So, when the shell is struck by predators, cracks either stop in the glue, or deflect between the layers of chalk, and the snail survives. By bringing this approach, my work will enable the materials in fusion power plants to withstand even more extreme environments and thus enable them to operate for longer, which will in turn decrease the cost of fusion energy.

Fusion is the process that powers the Sun, and if it can be reproduced here on Earth it would solve one of the biggest challenges facing humanity - plentiful, safe, sustainable power to the grid. For fusion to occur requires the deuterium and tritium (DT) mix of fuels to be heated to ten times the temperature at the centre of the Sun, and confined for sufficient time at sufficient density. The fuel is then in the plasma state - a form of ionised gas. Our CDT explores two approaches to creating the fusion conditions in the plasma: (1) magnetic confinement fusion which holds the fuel by magnetic fields at relatively low density for relatively long times in a chamber called a tokamak, and (2) inertial confinement fusion which holds the fuel for a very short time related to the plasma inertia but at huge densities which are achieved by powerful lasers focused onto a solid DT pellet. A main driver for our CDT is the people that are required as we approach the final stages towards the commercialisation of fusion energy. This requires high calibre researchers to be internationally competitive and win time on the new generation of fusion facilities such as the 15Bn Euro ITER international tokamak under construction in the South of France, and the range of new high power laser facilities across Europe and beyond (e.g. NIF in the US). ITER, for example, will produce ten times more fusion power than that used to heat the plasma to fusion conditions, to answer the final physics questions and most technology questions to enable the design of the first demonstration reactors. Fusion integrates many research areas. Our CDT trains across plasma physics and materials strands, giving students depth of knowledge in their chosen strand, but also breadth across both to instil an understanding of how the two are closely coupled in a fusion device. Training in advanced instrumentation and microscopy is required to understand how materials and plasmas behave (and interact) in the extreme fusion conditions. Advanced computing cuts across materials science and plasma physics, so high performance computing is embedded in our taught programme and several PhD research projects. Fusion requires advances in technology as well as scientific research. We focus on areas that link to our core interests of materials and plasmas, such as the negative ion sources required for the large neutral beam heating systems or the design of the divertor components to handle high heat loads. Our students have access to world-class facilities that enhance the local infrastructure of the partner universities. The Central Laser Facility and Orion laser at AWE, for example, provide an important UK capability, while LMJ, XFEL and the ELI suite of laser facilities offer opportunities for high impact research to establish track records. In materials, we have access to the National Ion Beam Centre, including Dalton Cumbria Facility; the Materials Research Facility at Culham for studying radioactive samples; the emerging capability of the Royce institute, and the Jules Horowitz reactor for neutron irradiation experiments in the near future. The JET and MAST-U tokamaks at Culham are key for plasma physics and materials science. MAST-U is returning to experiments following a £55M upgrade, while JET is preparing for record- breaking fusion experiments with DT. Overseas, we have an MoU with the Korean national fusion institute (NFRI) to collaborate on materials research and on their superconducting tokamak, KSTAR. The latter provides important experience for our students as both the JT-60SA tokamak (under construction in Japan as an EU-Japan collaboration) and ITER will have superconducting magnets, and plays to the strengths of our superconducting materials capability at Durham and Oxford. These opportunities together provide an excellent training environment and create a high impact arena with strong international visibility for our students.