Three, two, one ... We have plasma! Inside the European JET Tokamak, both during (right) and after operation. Photo: CCFE, JET. At the core of fusion science is plasma physics. At extreme temperatures, electrons are separated from nuclei and a gas becomes a plasma—an ionized state of matter similar to a gas. Composed of charged particles (positive nuclei and negative electrons), plasmas are very tenuous environments, nearly one million times less dense than the air we breathe. Fusion plasmas provide the environment in which light elements can fuse and yield energy. Three conditions must be fulfilled to achieve fusion in a laboratory: very high temperature (to provoke high-energy collisions); sufficient plasma particle density (to increase the likelihood that collisions do occur); and sufficient confinement time (to hold the plasma, which has a propensity to expand, within a defined volume). In ITER, fusion will be achieved in a tokamak device that uses magnetic fields to contain and control the hot plasma. The plasma particles are heated—that is, sped up—by different types of auxiliary heating methods. The fusion between deuterium and tritium (DT) nuclei produces one helium nucleus, one neutron, and great amounts of energy.
More than 99% of the Universe exists as plasma, including interstellar matter, stars and the Sun. Examples of plasmas on Earth are: neon tubes, lightning, the northern lights (aurora borealis), and the glow of plasma televisions. The helium nucleus carries an electric charge which will be subject to the magnetic fields of the tokamak and remain confined within the plasma, contributing to its continued heating. However, approximately 80 percent of the energy produced is carried away from the plasma by the neutron which has no electrical charge and is therefore unaffected by magnetic fields. The neutrons will be absorbed by the surrounding walls of the tokamak, where their kinetic energy will be transferred to the walls as heat. In ITER, this heat will be captured by cooling water circulating in the vessel walls and eventually dispersed through cooling towers. In the type of fusion power plant envisaged for the second half of this century, the heat will be used to produce steam and—by way of turbines and alternators—electricity. In terms of sheer scale, the energy potential of the fusion reaction is superior to all other energy sources that we know on Earth. Fusing atoms together in a controlled way releases nearly four million times more energy than a chemical reaction such as the burning of coal, oil or gas and four times more than nuclear fission.
Fusion is the process that takes place in the heart of stars and provides the power that drives the universe. When light nuclei fuse to form a heavier nucleus, they release bursts of energy. This is the opposite of nuclear fission – the reaction that is used in nuclear power stations today – in which energy is released when a nucleus splits apart to form smaller nuclei. To produce energy from fusion here on Earth, a combination of hydrogen gases – deuterium and tritium – are heated to very high temperatures (over 100 million degrees Celsius). The gas becomes a plasma and the nuclei combine to form a helium nucleus and a neutron, with a tiny fraction of the mass converted into ‘fusion’ energy. A plasma with millions of these reactions every second can provide a huge amount of energy from very small amounts of fuel. One way to control the intensely hot plasma is to use powerful magnets. The most advanced device for this is the ‘tokamak’, a Russian word for a ring-shaped magnetic chamber. CCFE’s goal is to develop fusion reactors using the tokamak concept.
With increasing concerns over climate change and finite supplies of fossil fuels, we need new, better ways to meet our growing demand for energy. The benefits of fusion power make it an extremely attractive option:
The United Kingdom’s fusion research programme is based at Culham Centre for Fusion Energy (CCFE) in Oxfordshire, the fusion research arm of the UK Atomic Energy Authority. The research is funded by the Engineering and Physical Sciences Research Council and by the European Union under the Euratom treaty. The UK contributes to fusion research in two main ways:
Its own fusion programme, centred on the MAST (Mega Amp Spherical Tokamak) Upgrade device. MAST Upgrade builds on the success of the original MAST tokamak (2000-2013) with major new capabilities in areas such as plasma stability and exhaust. The UK programme is also contributing to preparations for the international ITER project, and research on plasma physics and fusion materials and technology.
Operating JET – the Joint European Torus, the world’s largest tokamak and Europe’s flagship experiment. JET is situated at Culham, where CCFE operates it on behalf of fusion researchers around Europe via a contract between the European Commission and the United Kingdom Atomic Energy Authority.
Progress in fusion research
Researchers have overcome many of the scientific hurdles in fusion – developing a good understanding of how to control and confine the hot plasma of fuels. JET has produced a record-breaking 59 megajoules of sustained fusion energy over a five second period (the duration of the fusion experiment) using deuterium and tritium – the same fuel mix that will be used in future powerplants. During this experiment, JET averaged a fusion power of around 11 megawatts. Today’s tokamaks have high auxiliary power requirements to run the heating systems and energise the magnetic coils. However, research into reducing these requirements – notably through the use of superconducting magnets – is underway. The JET experiments are vital for the next large international experiment, ITER, and will also influence the design work of demonstration fusion powerplants, DEMO and STEP. CCFE is part of a worldwide research programme to show that fusion is viable. ITER will demonstrate the physics of controlling a power plant-scale fusion plasma. The challenge now is to develop the technology and engineering of tokamaks to capture fusion neutrons and produce electricity. This will prove fusion not only works as an experiment, but works economically on the scale of a power plant.
European fusion research is following a roadmap to achieve power generation around the middle of this century.
Beyond JET, the programme focusses on four main projects:
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