Fusion starts out from what seems a 'simple' nuclear reaction: take two atoms, mash them together and you get a lot of energy. Voilà, you now understand fusion. However, when you start thinking about exactly how you want to 'mash them together' you run into an issue, and another, and another, and... It turns out you need complex heating, and magnetic fields, and excellent reactor walls, etc. Mashing atoms together turns out to be more difficult than you'd think. Welcome to the world of nuclear fusion.
A nuclear reaction
Nuclear fusion has a very simple recipe. Take two light atoms and make them very hot. The atoms, or more precisely nuclei—because at fusion temperatures they will have lost their electrons—will fuse together and produce energy in the process. That is, if your atoms are light enough. Although you can in principle fuse any combination of atoms, some of them work better than others.
The most efficient process that we can use on Earth is the deuterium-tritium (DT) reaction, and it is thus the way to go for fusion. The process is shown in the picture. In step 1 a hot tritium (left) and a hot deuterium (right) nucleus collide. They fuse together and in step 2 two new products—a neutron (left) and a helium (right)—fly apart, even hotter than before.
At the temperature where fusion is possible, atoms can no longer keep hold of their electrons. The negatively charged electron flies off, leaving a positively charged ion. This combination of hot electrons and ions is what fusion scientists call a plasma. This plasma is so hot that it must stay well away from the reactor walls, because those would otherwise melt. To this end, the plasma is confined by strong magnetic fields. This makes sure that the heat from the plasma stays in the plasma. However, magnetised plasmas are fickle things. They are unstable, and experience a lot of turbulence. And there's the question of how to heat them. And... Put in short, there's a whole host of plasma problems that needs to be solved for fusion to work, and many different groups of scientists have been working on many different aspects for a long time already.
Even if we are able to successfully confine a 200-million-degree hot plasma, the materials that go into a fusion reactor are subject to the most extreme environment on Earth. They have to contend with huge heat fluxes, and super hot neutrons requiring extremely resilient materials that do not wear excessively under the high heatloads in a reactor. Meanwhile, superconducting magnets are sitting mere meters away and need to be cooled down to a couple hundred degrees below zero Celsius. The forces on these magnets are huge, putting big constraints on the construction around the reactor chamber. Whatever which way we look, fusion needs extreme materials.
Systems and control
A fusion reactor is a highly complex system and everything has to tick in perfect harmony. Systems are required to keep the plasma in place, and to control the fuelling and heating. The walls need to be cooled and all the heat has to go somewhere. The magnets need to get the right amount of current at the right time, while also being cooled. This and a million other things need to work in just the right way, requiring complex control mechanics as well as the mechanical and electrical infrastructure to support that. All disciplines come together as the plasma physicist needs to work with the control engineer, who needs to talk with the mechanical engineer that builds the actuator and the electrical engineer that builds the circuit. You get the gist.
And so much more...
And that's just one controller of one system that is a part of the reactor which is a part of the reactor building which sits on the site. Fusion is so extremely complex that every problem needs dozens of people in as many different disciplines to solve. This page does not even begin to scratch the surface of everything that goes into fusion. Whatever your particular interest, there is most likely an area of fusion that needs it.
Building a fusion reactor
Photo: ITER Organization/EJF Riche.
Building the most complex machine on Earth, as it suggests, is quite a difficult task. This video shows the different steps of building the ITER reactor chamber. All of this fits into the right half of the big building in the background picture. Building a functioning fusion power plant is not at all easy.