Author: Margaret Graham CEng FIET, Ion Cyclotron Resonant  Heating group leader, Culham Centre for Fusion Energy, Culham Science Centre How will the world’s ever-increasing energy demands be met once gas and oil are depleted? This is a dilemma that the world needs to solve before it is too late. While renewable energy sources, such as wind and solar power, are capable of providing a sizable chunk of future energy needs, it is clear that this will simply not be enough. Nuclear energy from fission power is often touted as a solution but is increasingly falling out of favour and, indeed, is itself not limitless, so what other alternatives are there? One potentially game-changing alternative is fusion energy. This is the mechanism that powers the sun and all other stars in the universe and, if harnessed, could offer the holy grail of energy: a safe, clean and abundant source of power.

Fusion energy

Every second, the sun fuses 620 billion tonnes of hydrogen into helium, releasing about 1x1026 (more than a billion billion) Watts of power. The enormity of the sun’s output is due in part to the huge quantity of energy released by each fusion reaction, but is also linked to its gargantuan size. In fact, the power density of the sun is, at around 1 kW/m3, comparable with the heat generated in an active compost heap. For fusion to be viable on earth, on practical volumetric scales, we need to create an artificial star with a power density about a thousand-fold higher. To this end, fusion research is currently focused on the deuterium-tritium (D-T) reaction, which enables power densities around 1 MW/m3 to be achieved, albeit at temperatures ten times hotter than the center of our sun. The challenge of creating such a ‘star’ here on Earth is one that dozens of nations are pursuing, with the scientists and engineers working at JET (Joint European Torus), the world’s largest fusion device, leading the way. Since its first experiments in 1983, research at JET has focused on working out how a commercially viable fusion power station might be realised and the steps that are needed to get there. While this sounds simple, it is actually far from straight-forward. JET uses a magnetic fusion device, called a tokamak, to contain and heat the fusion fuel (the deuterium (D) and tritium (T)), which are held in the form of a highly ionised gas known as plasma. [caption id="attachment_17246" align="alignright" width="367"]New Picture D-T reaction[/caption] The D-T reaction (right) is favoured as it produces the highest energy output at plasma temperatures that are possible to attain (and confine) with today’s technologies. When D and T nuclei fuse, a helium ion and a high speed neutron is released. For each reaction 17.5 MeV of energy is released. Compared to the quantity of fuel, this is a truly vast amount of energy; each gram of reacted D and T is equivalent to burning ten million grams of coal or gas! With this vast power potentially available, how far has fusion come?

Progress and challenges

A useful fusion figure of merit is the triple product, essentially a measure of a device’s ability to contain enough hot fuel. This is defined as the product of the plasma density, temperature and confinement time. Since the 1960s, the fusion triple product has almost doubled every two years. This rate is comparable with the often hailed progress in computing known as Moore’s Law, which states that processing speed or power doubles every two years, and highlights that research in fusion energy has seen a formidable and sustained progress. New Picture Another measure of fusion progress is given by the ratio of input power to fusion output power, Q. A self-sustaining fusion reaction, where a plasma heats itself by fusion power alone, corresponds to infinite Q. However, reaching infinite Q, also termed achieving ignition, is not a necessary condition for a practical fusion reactor. A value of Q somewhere upwards of 20 is generally accepted as sufficient. The condition of Q = 1 is referred to as breakeven, and marks the point at which the energy released matches the input. During the 1997 D-T experiments, JET produced the current world record of 16 MW of fusion power, which equates to Q = 0.6. Scientists and engineers at JET are looking forward to challenging this record later in this decade with further D-T experiments planned. In light of this amazing progress, one might ask: “Why has a commercial fusion plant not yet been built?” To answer this, we must try to understand some of the many engineering challenges that have to be overcome in order to build a power plant. The most obvious of these challenges is the ability to contain a plasma ten times hotter than the centre of the sun. After that come the challenges of actually creating those temperatures, keeping the plasma stable and dealing with the material issues associated with neutron bombardment. Beyond these basic issues, there is also a requirement to measure the plasma properties. The diagnostic challenges associated with determining the conditions inside a fusion reactor are considerable. Most importantly, any fusion plant needs to meet the Lawson criteria (related to the triple product), which states that the critical temperature must be maintained for a sufficiently long confinement time at a high-enough ion density in order to obtain a net yield of energy. While JET regularly achieves temperatures over 150 million degrees and leads ground-breaking research into ways of maximising plasma density and confinement, it is not (and was never designed to be) equipped to fully meet the Lawson criteria. Additionally, any commercial fusion power station needs to generate electricity from the products of the fusion reaction; the high energy neutrons. Currently envisioned methods of doing this involve slowing down the neutrons in a lithium blanket, which in turn heats water to drive steam turbines in a conventional way.

Fusion roadmap

JET Machine from Crane WalkwayWhile there has been outstanding progress in fusion, there is still the need to drive an aggressive programme to deliver a fusion power station. This need stems not only from the predicted increases in near-future energy demands and the current requirements to minimise green-house gases, but also from the very real need to maintain momentum within fusion research and its supporting technologies. Within the European Union’s Horizon 2020 Research and Innovation programme a fusion roadmap (1) has been developed that will deliver funding over the seven years from 2014 to 2020. In this roadmap, a ‘fast track’ approach to fusion energy research has been adopted, which is based on constructing and exploiting three facilities:
  • ITER, the next stage device in the path to magnetically confined fusion power, which is similar in design to JET;
  • The International Fusion Materials Irradiation Facility (IFMIF), a centre for fusion materials research, run in parallel with ITER;
  • DEMO, a prototype demonstration reactor.
The first of these, ITER, is an international project involving 35 nations including the European Union, India, Japan, the People's Republic of China, Russia, South Korea and the United States of America. ITER is currently being built in Caderache in the south of France. It is twice the linear size and 10 times the volume of JET and will have superconducting magnets to allow longer pulse operation. ITER is expected to achieve 500MW of fusion power at Q = 10, a value which will correspond to a significant fraction of the heating being driven by the fusion reaction itself. ITER will also have the capability to accommodate some prototype lithium blanket modules. These modules will be used to research and engineer a system that can provide a means to convert the energy from the high-energy neutrons to electricity and also provide a mechanism to ‘breed’ tritium which can be used to refuel the reactor. Alongside ITER, IFMIF is essential in the work needed to progress fusion since the materials required for a fusion reactor will need to withstand a uniquely extreme and harsh environment. Indeed, materials research is one of the most vital engineering requirements needed to realise a successful fusion reactor and IFMIF will be the only facility that can perform realistic tests with fast-fusion neutrons. Finally, in order to further the progress made in these two key areas, a demonstration reactor will be essential in order to bring everything from ITER and IFMIF together. DEMO, the demonstration power plant, will provide the final validation, and will establish fusion as a practical and desirable option for the world’s electricity demands.

Conclusion

Fusion has clearly made significant strides over the years and is definitely a contender in the race to find a solution to the world’s future energy demands. Fusion power is tantalisingly attractive, not least because the fuels are virtually limitless and the by-products have no long term negative impact on our planet (i.e. no green-house gas emissions, no long term radioactivity), but also because it is an intrinsically safe process. However, much is still to be achieved. Many key issues remain unresolved and many more challenges are, with certainty, yet to be discovered. The EU roadmap shows what has already been achieved and describes, through future areas of focus, the outstanding issues still to be addressed, and also provides a timeline and plan that aims to realise a commercially viable fusion power station by the middle of the 21st century. The next few decades will be critical in the development of fusion technology, which genuinely has the potential to change the path of human history. The author would like to thank Dr Joanne Flanagan and the Culham Public Relations team for their outstanding support in writing this article. Reference: (1) ‘Fusion Electricity - A Roadmap to the realisation of Fusion Energy.’ 
Margaret Graham CEng FIET is an electrical/electronic and radio frequency (RF) engineer working at the fusion research Culham Science Centre in Abingdon. On completing her Higher National Certificate in electrical and electronic engineering in 1986, she started her career as a technician at Torness Power Station in Scotland before moving to Culham in 1988. Graham’s engineering career developed over the years as she graduated with a Batchelor of Engineering (honours) degree from the Open University and a Masters of Science in high power radio frequency science and engineering from Lancaster University. She was promoted to Ion Cyclotron Resonant Heating group leader in 2008, became chartered in 2011 and a Fellow of the IET in 2013. As Ion Cyclotron Resonant Heating group leader, Graham is responsible for the 23 to 57MHZ, 32MW RF system which is used to heat the plasma at the world leading JET fusion project in Oxfordshire. She also works with the IET as an industrial representative, a professional registration advisor, is the scheme administrator for CCFE’s accredited professional development scheme and is a member of the IET registration and standards committee.