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Nuclear fusion, although currently industrially unviable, is a promising source of energy with the potential to meet the deficiency left by the future depletion of fossil fuels. Scientists from across the world are collaborating to turn this theory into reality.

Why is a fusion reaction so tremendously difficult to initiate? The most favourable fusion reaction that we are aware of begins by smashing two hydrogen nuclei into each other. Since nuclei are positive, there is a huge electrostatic repulsion to overcome (the Coulomb barrier). On the other hand, a typical fusion reaction begins with firing a neutron into a uranium-235 nucleus. A neutron has no charge, so there is no electrostatic repulsion between the two reactants. In fact fission reactions end up occurring so rapidly that we require moderators and control rods in fission reactors to slow down the chain reaction.

The Coulomb barrier is a very significant hindrance. Due to this, fusion reactions require temperatures of over 150 million degrees Celsius in order to initiate in order to provide the nuclei with enough energy to overcome the Coulomb barrier. This is a monumental feat to achieve, bearing in mind that the temperature in the core of the Sun is only approximately 15 million degrees Celsius. Although fusion also occurs within the Sun, the minimum temperature can be much lower since it is supported by the Sun’s immense gravitational pressure.

Surprisingly, 150 million degrees Celsius has not been too difficult to achieve. What does prove a problem though is being able to harness more energy from fusion than it takes to initiate it.

There are currently two major approaches to fusion reactors. The first, which I will be focusing on today, is magnetic confinement fusion (MCF). The fuel is first heated to an extremely high temperature, causing electrons to be stripped off its nuclei, and forming a plasma. A plasma, commonly termed the fourth state of matter, is in essence a cloud of high-energy nuclei and electrons. Since a plasma comprises free-moving, charged particles, it can be manipulated using magnetic fields. MCF attempts to confine the plasma inside a chamber so that the plasma does not rapidly cool down. Any energy released is absorbed through the walls of the chamber.

There are several devices that attempt to utilise MCF, but the most favourable one to date is certainly the tokamak, first designed by Soviet physicists in the 1950s. The word is a transliteration of the Russian word токaмак, an acronym of a Russian phrase which can be translated to toroidal chamber with an axial magnetic field. Correctly so, the tokamak is a doughnut-shaped vacuum chamber with magnetic fields. A circular path for the plasma to traverse is ideal as the plasma is not able to escape.

Tokamak.jpg
Image: ITER

MCF is arguably making the most progress in the field of fusion power. A new fusion experiment, ITER, is currently under construction in Saint-Paul-lès-Durance in the south of France, and its tokamak will be the largest in the world. An acronym for the International Thermonuclear Experimental Reactor, and also meaning ‘the way’ in Latin, ITER is the result of the collaboration between thousands of scientists and engineers from seven member entities: China, the European Union, India, Japan, Russia, South Korea, and the USA. Let’s hope that it will pave ‘the way’ for the future of nuclear fusion!

Yanhao

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