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What is the fusion energy gain factor?

how much progress have we made towards using fusion to generate electricity? Here, we introduce the concept of the fusion energy gain factor.
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Hello. My name is Howard Wilson. I am a Professor of Plasma Physics at the University of York, and have recently returned from a secondment to UK Atomic Energy Authority, where my roles included the interim Director of the new £222M STEP fusion reactor design programme. I am going to tell you a bit more about the status of fusion, focusing in particular on the tokamak. This is a fusion energy device, which holds the hot fusion fuel in the plasma state, keeping it away from the walls by a toroidal configuration of magnetic field. The international fusion community is presently constructing the ITER tokamak in the South of France, which is the biggest international science project on Earth.
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Designed to deliver ten times more fusion power than the power injected into the plasma to reach fusion conditions, it will answer the remaining science questions and address much of the technology required to build the first demonstration power plant. This is an exciting prospect, that is the culmination of decades of research. The progress to ITER can be illustrated with this figure. Along the x-axis we plot the temperature. The centre of the sun is here, at 10 Million kelvin. Up the y-axis we plot the triple product - the product of the plasma density, its temperature and something called the confinement time, which measures how good the magnetic field is at containing the plasma.
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The curves labelled Q=1 (which is where fusion power equals the plasma heating power injected) and ignition indicate the values of temperature and triple product required for a commercially viable fusion power plant. Our story begins with the small Russian tokamak, T3, back in the 1960’s. A team of British scientists travelled to Moscow to make the first measurements of temperature in a tokamak plasma using a new laser-based system called Thomson Scattering. These measurements showed the huge potential of the tokamak, but T3 was still a factor of 100 too low in temperature and a factor of 10,000 off in triple product, compared to where a commercial fusion reactor must operate.
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Nevertheless, excitement about the very high temperatures achieved led to a number of tokamaks to be built outside the Soviet Union during the 1970’s and 80’s, achieving still higher plasma temperatures, and higher triple product. These acronyms are the names of the different tokamaks. You can see how rapid progress was made through the 80’s – especially with the operation of the European tokamak JET in the UK, which is still the world’s most advanced fusion energy facility. During the 1990’s, both JET and the Japanese tokamak, JT-60U, finally demonstrated that Q=1 – scientific breakeven – is possible.
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To go further with this kind of tokamak design required a still bigger device to achieve the necessary triple product – and that led to the design and construction of ITER. JET underwent a huge programme of work to replace all of its internal tiles to be of the same materials as planned for ITER – beryllium and tungsten. This was all done with state of the art robotics systems – again, a key fusion technology. If ITER is successful – and there is much scientific evidence to indicate it will be – we will need to push on towards the design of the first demonstration fusion power plant. Several countries are thinking seriously about this.
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In Europe the focus is on a demonstration power plant that is a scaled up version of ITER. The strategy is to build an irradiation facility – called IFMIF-DONES – that will test materials in a fusion neutron environment, and will operate alongside ITER and, in parallel, design the demonstration power plant, called DEMO, that will likely operate from the 2060’s. The UK is a strong partner in this European programme but, with the urgency of climate change, is asking the question, “Can we deliver fusion faster and with lower capital cost?” This is a goal of the STEP programme – the Spherical Tokamak for Energy Production.
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Specifically STEP is assessing the feasibility of delivering a prototype commercial reactor by 2040 that is capable of providing net electricity, aiming to deliver an initial design by 2024. The spherical tokamak is a variant of the tokamak design that is more compact and can support a much higher plasma pressure for a given magnetic field. It is, however, a relatively new concept and does not have the huge plasma database that we saw earlier for the conventional ITER-like tokamak. A key input to the STEP programme, therefore, is not only to learn from our participation with Europe in ITER and IFMIF-DONES, but also to perform experiments on a new spherical tokamak called MAST Upgrade that is just starting to operate here in the UK.
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Together with advanced computer simulations and theory, and the US sister spherical tokamak NSTX Upgrade, that will also operate in the next few years, this will provide the scientific basis for a decision on an accelerated fusion programme via STEP.

So how much progress have we made towards using fusion to generate electricity? Here, we introduce the concept of the fusion energy gain factor, normally written as (Q). (Q) is the ratio of power generated via fusion to externally provided heating power.

(Q=1) denotes scientific breakeven. Every bit of energy we produced would need to be pushed back into the plasma to keep it hot enough for fusion to take place. At (Q=1), the reaction is not really self-sustaining as we will never be able to capture all of the energy from fusion with perfect efficiency and push it back into the plasma. This is why we call it ‘scientific’ breakeven. It is a really important step along the journey, but it does not lead to a functioning reactor for energy production.

A commercially viable fusion reactor must have (Q) much bigger than 1. We need plenty of additional energy output to allow for losses and inefficiencies so that we can both keep the plasma burning and produce large amounts of electricity to deliver to the grid.

(Q=infty) corresponds to ignition, meaning no external heating power needs to be applied to maintain fusion conditions. This occurs when the energy deposited by helium nuclei produced in the fusion reaction alone is sufficient to keep the plasma at fusion conditions. We would still need to keep the plasma confined using external fields, we just don’t need to keep topping up the energy to keep things going.

Now that you have heard more about our journey towards fusion are you optimistic that we will be able to provide commercial fusion power in the future? 

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