Now that we’ve seen from the bottom up how nuclear fusion works, let’s talk about fusion reactors. There are a lot of different kinds of reactors that can make fusion happen, but for the sake of simplicity, we’re only going to talk about one: a magnetic confinement fusion reactor. As we’ve seen in a previous part, magnetic confinement is necessary to keep the plasma from expanding and losing its energy. These types of reactors are the most common and have made a lot of advancements. The first commercial reactors will probably be magnetic confinement reactors, however if there is a sudden discovery regarding a different type of fusion reactor, things could change pretty fast. The reason I’m saying that the first commercial reactors are probably going to be of this kind is because of the fact that ITER, the largest nuclear fusion test reactor, is a magnetic confinement fusion reactor.
Within this type of fusion reactors there are 2 designs that show much more potential than the rest of them. On one side we have the tokamak and on the other side there’s the stellarator. The word “tokamak” is a contraction of the Russian words toroidalnaja, kamera, magnitnaja en katoesjka, which more or less translates to toroidal chamber with magnetic coils. The device was invented in 1950 by the Russian physicists Igor Tamm and Andrei Sakharov, with the purpose of studying plasma’s and eventually achieving fusion. The shape of the plasma chamber is the same as that of a donut, which is toroidal. A plasma of a 150 million degrees is so extremely hot that no material known to man can resist the heat, this is why the device uses magnetic coils to keep the plasma in the center of the torus. Those coils generate a homogenous magnetic field in the center, so at first this seems like the ideal solution (Figure 12).
Ions follow helical paths around magnetic field lines, they spiral around them. This is not efficient. Too many particles can still escape the magnetic field, which results in significant energy loss. To be as efficient as possible, we need the magnetic field to be helicoidal. This kind of field is acquired when we generate another magnetic field perpendicular to the toroidal field – a poloidal field (Figure 13).
To generate this field a current is induced in the plasma itself. The resulting magnetic field is optimal for keeping all the particles trapped in the center:
The toroidal magnets (Figure 15; Blue) are built around the vacuum chamber (Figure 15; Yellow), in which the plasma is confined without reaching the walls. These electromagnets are cooled to an extremely low temperature (around 4 Kelvin) to make them superconductive. This means that they can conduct electricity with almost no resistance, so that the energy is used as efficiently as possible and the magnetic field that’s generated is as strong as possible. In a superconducting state, the metal can also conduct a much higher current. A metal becomes superconductive at cryogenic temperatures, below 150 Kelvin (-123° C). But the colder, the better. Modern reactors cool their conductors to about 4 Kelvin (-269° C). The vacuum chamber is a hermetically sealed space in which the plasma is confined. It keeps the plasma stable and works as a barrier against radioactivity. There’s a lot of sealed openings in the chamber walls for systems like the heating mechanisms and controlling mechanisms.
The inside of the vacuum chamber is covered by a “blanket” (Figure 15, Red). This protects the surrounding components from extreme heat and fast-moving neutrons. The neutrons are slowed down by the blanket and their kinetic energy will be converted to heat. In nuclear fusion power plants this heat will be used to drive a steam turbine, which will then produce electricity. Although nuclear fusion seems very futuristic, to actually generate electricity from it we still have to rely on a relatively old technology – the steam turbine. This blanket consists of a lot of different replaceable panels. The panels become radioactive because of the highly energetic neutrons that collide into them. The good thing about fusion though is that the radio-isotopes that are formed have a short half-life, which ensures that we can reuse the panels after about 150 years without danger of radiation.
On the bottom of the vacuum chamber the divertor is located (Figure 15, Blue). This device extracts the helium ashes and the heat produced by the fusion reactions, keeps the plasma clear of impurities and has a protective function against radioactivity – just like the blanket. The divertor also becomes incredibly hot and will further power the steam turbine.
Another critical part is the cryostat. This device makes sure that the surroundings are extremely cold, which is important for the efficiency of the superconducting electromagnets. It also makes it easier to maintain the necessary vacuum.
To reach the wanted density in the center of the torus, fuel particles are constantly injected into the plasma. This is done by a device called the pellet injector. As the name says, it injects small pellets of frozen tritium and deuterium – again at cryogenic temperatures – into the plasma. The pellet injector also maintains another function. A plasma is inherently unstable, and if it is not sufficiently controlled this can lead to heavy turbulence. When these pellets are shot into the plasma, a little turbulence is created – controlled turbulence. With creating controlled turbulences we essentially stabilize the plasma, which decreases the chance of heavy turbulence occurring.
The central part of a tokamak is a transformer coil. This coil induces a current in the plasma and heats it up to about a third of the necessary temperature through Ohmic heating (Figure 16). This coil or solenoid acts like the first coil; if a current goes through it generates a magnetic field that induces a current in the plasma – the second coil of the transformer.
Thanks for reading part 7 of this thread on nuclear fusion. This concludes the technical chapter. In the next few posts, we're going to be focusing on why we need nuclear fusion. We'll also compare fusion to other sources of energy to bring in some perspective. After that, we'll be talking about the future of nuclear fusion, using past experiments and progress as a reference. If you haven't read the first few posts, make sure you read them first!
