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Cambridge University Science Magazine
Nuclear fusion in the core of stars has allowed them to shine brightly for billions of years. About a century ago, the idea of producing energy by creating a miniature star on Earth sounded like something straight out of science fiction. Today, nuclear fusion has become the holy grail of limitless clean energy. While mankind has gotten pretty good at extracting energy from the chemical bonds in fossil fuels, we have reached a point where its by-products such as carbon dioxide are threatening to warm the planet beyond a point of no return. Hence, the development of renewable clean energy to cure our carbon addiction is more pertinent than ever; fusion energy could be the best antidote.

Nuclear fusion essentially aims to recreate the condition of the Sun on Earth. The Sun produces energy by smashing hydrogen atoms together in its core under intense heat and gravity to form nuclei of helium, in what is called the proton-proton reaction. This reaction emits an enormous amount of energy, which powers the entire solar system.

In laboratories, the ingredients for fusion are not just any hydrogen, but specific isotopes with extra neutrons, namely deuterium and tritium. For nuclear fusion to occur, the nuclei must have enough energy to overcome the strong electrostatic repulsion between protons to get close enough for the attractive but short-ranged strong nuclear force to dominate and snap the nuclei together. Hence, we have to subject the hydrogen gas to immense pressures and extreme temperatures of up to 100 million degrees Celsius (6.5 times hotter than the core of the Sun), creating plasma in the process. The way to do it essentially involves squeezing and heating. One method is electromagnetic confinement that uses strong magnetic fields and microwaves together with neutral beam injection, which shoots highly accelerated neutral particles at the gas. In short, it is a microwave on steroids. Another method is inertial confinement which uses pulses from superpowered laser beams to subject the surface of a deuterium-tritium fuel capsule to tremendous pressure and temperature, causing it to implode and achieve fusion conditions. Once the plasma is created, it is suspended in space by strong magnetic fields created by superconducting electromagnets. The fusion reaction releases energetic neutrons, carrying 80% of the energy from the reaction with them. These neutrons are what is harvested to generate electricity.

Currently, there are two main models of nuclear fusion reactors: the Tokamak and the Stellarator, each with their unique set of challenges and advantages. The key difference between them is their geometry. The Tokamak is toroidal in shape and symmetrical. On the other hand, the Stellarator has a complex helical twisted structure. Most fusion reactors now are Tokamaks. The presently largest one is the Joint European Torus (JET) in Oxfordshire. Others include the ongoing construction of the International Thermonuclear Experimental Reactor (ITER) in southern France and Experimental Advanced Superconducting Tokamak (EAST) in China. The most advanced stellarator is the Wendelstein 7-X (W7-X) in Germany. Fusion research is thus an ongoing international effort.

The ITER, which is a multinational project involving 35 nations, aims to create the world’s largest and most powerful reactor. It is twice as large as JET and aims to produce ten times the input energy. As impressive as it may seem, the immediate goal of ITER is not energy production, and this 22 billion dollar project is simply a grand science experiment to gather enough information to allow the next generation to build the DEMOnstration Power Plant (DEMO), which is industry-driven with the aim of commercialisation by 2050. To give an idea of the technological capability going into fusion reactors: large superconducting electromagnets are cooled with liquid helium to temperatures just shy of absolute zero, while just metres away lies the intensely hot plasma. The reactor has to be built to withstand the largest temperature gradient in the known universe.

Nuclear fusion, if realised, has many advantages over current alternative sources of energy. It is convenient to compare fusion with fission power. Fission is tainted with poor public acceptance as various incidents in the past, particularly the ones of Fukushima and Chernobyl, have caused fear among the general public. In addition, it is plagued with problems of radioactive nuclear waste disposal and risk of nuclear proliferation. On the other hand, fusion does not run the risk of a meltdown because if anything happens, the plasma expands and cools within seconds, forcing the reaction to stop. Put simply, it is not a bomb. It also releases four times more energy per unit mass than fission and four million times more compared to coal and gas. The only radioactive substance produced is residual radioactivity in the structural material which is short-lived and can be minimised with better design.

However, fusion has its fair share of technical and practical challenges. For fusion to be even viable in the first place, the output energy has to exceed the input heating energy in what is termed ‘break-even’. The ratio of fusion power to input energy is a value called the Q factor. A desirable Q factor would be greater than one. The best ratio now is only 0.7, achieved by the National Ignition Facility (NIF) in August 2021. However, the fusion reaction only lasted for several billionth of a second. Clearly, another critical criteria is that energy production has to be sustained for longer times. The current record is held by JET where 59 MJ was produced over five seconds in February 2022. To give the numbers some meaning, 59 MJ is only sufficient to boil 60 kettles worth of water. Furthermore, a lack of tritium in nature poses a major barrier to its sustainability. While deuterium is abundant in seawater, tritium only occurs in trace amounts in nature, mainly produced by the effect of cosmic rays on the outer atmosphere. It is also difficult to artificially produce, making it incredibly expensive, driving the cost of fusion up. Making matters worse, most materials cannot withstand irradiation by energetic neutrons and suffer microstructural damage, which necessitates regular repair and replacement, again driving costs up. Also, absorption of tritium by the material in the reactor wall results in fuel loss. In the worst case, the fusion reactor could run out of fuel before it even gets started. Truth be told, it is evident that fusion is still far from fruition. As a result, there are many opposing voices to fusion, pointing out that it is nothing but science fiction. As basic research, it has value; but, to sell it as a technology that can solve the world’s energy problem is just deceptive.

Nuclear fusion research is an expensive gamble. Proponents of fusion want to turn the idea into reality and put forth the argument that ‘we have it when we need it’. Meanwhile, proponents of fission want to upgrade existing fission technology and change public perception, and climate advocates urge us to decarbonise with all the resources we have now before it's too late. At the end of the day, it is all a matter of time-scale. Renewable energy and nuclear fission are short to medium-term strategies to reduce reliance on fossil fuels. According to Kardeshev, who famously came up with the Kardashev scale, mankind would eventually grow out of breaking mere chemical bonds as it becomes insufficient to satiate our ever growing need for energy. It is then that our endeavour to harness the power of stars will bear its fruits.

Shikang Ni is a first-year undergraduate student studying Physical Natural Sciences at Wolfson College. Illustration by Caroline Reid.