Did you ever wonder how the sun can warm you or even burn your skin from a distance of about 150 million kilometres? The answer is relatively simple. Nuclear fusion is so powerful that you can feel the energy released by it from the core of the sun at such vast distance.
The human endeavour of understanding the process that keeps the sun burning started in 1920 with Arthur Eddington, who suggested that the fusion of hydrogen to helium could be the crucial mechanism . It took another brilliant mind, Ernest Rutherford, to actually realise the fusion reaction of deuterium to helium in the lab and thereby determine the enormous amount of energy released by the reaction .
Enormous can be taken quite literally here – nuclear reactions typically release one million times more energy per reaction than chemical reactions, for example combustion. The primary resources for current nuclear fusion technology are deuterium, which is naturally occurring in seawater (0.033 g/l) and lithium, abundant in the crust of the Earth in the form of minerals/rock formations. Lithium is used to breed the reaction partner for deuterium, namely tritium. To put this in perspective, the electricity gained via fusion from the amount of lithium in a typical laptop battery and the deuterium from half a bathtub of water is equivalent to the electricity gained from burning 40 tons of coal .
The stars’ recipe for fusion
There is one giant hurdle on the path to making two light nuclei fuse together and to harvesting the released energy – Coulomb’s law. This describes how electrically charged particles attract or repel each other in dependence on their distance. Since we want to fuse atomic nuclei, which always carry a positive charge, they repel each other more strongly as they approach one another. The way to overcome this so-called Coulomb barrier is by forcing the nuclei to collide with tremendous speeds. Speed on the atomic scale is nothing more than temperature.
As much energy as is released by one reaction, it is still not enough to power a city, let alone a star. Hence, there is another important quantity, namely the collision rate or collision probability. If the atomic nuclei move through a dense sea of other nuclei at high temperature, the collision rate is accordingly high and many reactions occur within a given time period.
In summary, a star like our Sun achieves nuclear fusion by having extremely high temperatures within its core of about 15 million Kelvin  plus having an immense pressure of about 265 billion times the Earth’s atmospheric pressure. The product of this temperature and particle density, which is proportional to the pressure, is large enough over a certain period of time to achieve a significant rate of fusion reactions.
Just mimicking the core of a star – no big deal, right?
Can we realise the conditions present in the core of the Sun here on Earth? Well, for either the temperature or the pressure we can, just not both at the same time over a long enough period – yet. Two paths have crystallised out of the conditions that are required to achieve fusion.
The first is creating the immense pressures and temperatures with a shockwave generated by pulsed lasers, called inertial confinement fusion . The inertial confinement fusion concept is still far from net energy generation due to the huge power consumption of the laser system of about 500 TW at maximum  for an energy output of about 1.8 MJ. In contrast, the fusion energy gain is only about 15 kJ, representing 0.8% of the input energy. The goal is, of course, to yield an amount significantly higher than the input energy.
The other path to achieve the conditions required for fusion is called magnetic confinement fusion. With this path, the temperature and time period are the focus. The particle temperature needed and achieved in magnetic confinement devices is ten times higher than in the core of the sun. This is because magnetic confinement fusion devices typically operate under vacuum conditions with a pressure of 5 ×10-6 mbar in the reaction chamber .
Now imagine something that is hotter than the core of the sun in contact with stone, metal, fireproof ceramics or any other known material. Exactly – it will just be vaporised. That is why these extremely hot particles are confined with a powerful magnetic field so that they never actually touch any wall.
The technically most mature magnetic confinement fusion concept is the tokamak – a doughnut-shaped reaction chamber with two kinds of magnetic coils confining the hot particles, i.e. the plasma. This was invented by Soviet physicists in the 1950s.
Toroidal magnetic field coils compress the plasma particle trajectories into the inner volume of the reaction chamber, while a central solenoid is discharged to induce a current into the plasma via the induction principle. This current is necessary to twist the particle trajectories in order to prevent a slow drift of the particles out of the confined volume.
The heating of the plasma is realised by neutral particle beam injection and radio-frequency heating . The current world record of fusion energy output in relation to the input energy was achieved by the Joint European Torus (JET) with about 60% in 1997 .
Another more elegant, yet less technically mature, magnetic fusion confinement concept is the stellarator architecture. With the stellarator, the need for the central magnetic field coils is eliminated by replacing the simple toroidal field coils with complex 3D shaped coils .
The viability of the stellarator architecture is currently being investigated with the Wendelstein 7-X facility in Greifswald, Germany. Stellarators are less technically mature due to the fact that they are not designed for deuterium-tritium fusion reactions yet since they first need to prove that the magnetic confinement is sufficient to pursue this concept further.
Watch a short video about Wendelstein 7-X, tokamaks’ rebellious cousin:
From seawater and rocks to your power outlet
As you can see, we have come at least very close to the breakeven of energy output versus input with the tokamak architecture. However, for a fusion power plant feeding electricity into the grid, a few to about ten times the input energy is needed to achieve economic viability.
To prove that this is possible, the experimental fusion reactor ITER (Latin for “the way”) is currently under construction in France. Its first plasma discharge is scheduled for 2025 and the full power phase is anticipated about 10 years later. ITER is close to a power plant scale reactor with an energy output of 500 MW .
Now you might wonder: Soviet scientists invented the concept in the 1950s, why on Earth does it take significantly more than half a century to achieve net energy gain with fusion? Especially, since nuclear fission was used to feed energy into the grid since the same decade the tokamak concept was invented. Let me tell you – it is an understatement to say that a plasma at 150 million Kelvin is a highly non-linear system.
In detail, any change of the plasma parameters (density, temperature) or magnetic field configuration can induce unwanted effects on other characteristics, for example, the heat deposition profile on the reaction chamber wall. Plasma physicists, materials scientists, and engineers work hand in hand to solve all challenges that controlled fusion as an energy source demands.
Rather recent technological breakthroughs like superconductors have enabled much stronger magnetic fields to allow for a better confinement of the particles. However, a strong confinement is only one piece of the puzzle.
The heat and helium ash generated from the fusion reactions need to be extracted. The term helium ash is used because deuterium and tritium fuse to form a helium nucleus that is of no further use in the plasma and needs to be removed in order to make room for new deuterium and tritium to react.
The heat exhaust of a power plant scale fusion reactor is among the critical issues and creates enormous challenges for materials scientists and engineers. Only materials with extremely high melting points, high thermal conductivity, and robustness against energetic particle bombardment are suitable for this application.
When eventually achieved, fusion will contribute to a sustainable and clean energy economy for the centuries to come.
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