Nuclear Fusion - How Does a Tokamak Work?
Using a nuclear reaction to produce tons of energy but no radioactive waste? What?! Sounds like magic I know, but really, it’s just physics!
By Raphaël Dacheux
Nuclear fusion is a reaction in which two (or more) atomic nuclei are combined to form one (or more) different atomic nuclei and subatomic particles (neutrons or protons). When two nuclei fuse, a small amount of their mass is converted into a large amount of energy, according to Einstein’s relation, E = mc². Indeed, the product’s total mass is lower than the reagent’s total mass. The mass difference corresponds to the energy produced.
Important to note: Fusion reactions are a natural process, fusion takes place in every star: the energy produced is the reason that they shine and produce heat. Fusion can occur only for light nuclei, weighing less than iron nuclei, but most elements can’t fuse unless they are in a star.
Wait, we already have energy from nuclear reactions, don’t we? Indeed! We are currently producing electricity with nuclear fission reactors. Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. Nuclear fission produces a good amount of energy but also involves high risks due to radioactive reagents, and nuclear wastes that are hard to manage after getting out of the reactor.
Fusion is a game changer:
- At equal mass, the fusion of light atoms releases energy four times greater than that of fission reactions.
- At equal mass, the fusion of light atoms releases energy almost four million times greater than that of a chemical reaction such as the combustion of coal, oil, or gas.
- No Risk of Core Meltdown: A Fukushima-type nuclear accident cannot occur in a fusion reactor, chain reactions are inconceivable for fusion.
- No long-lived high-level radioactive waste: all materials can be used or recycled (which means they are no longer radioactive) after 100 years maximum. They are low-activation materials.
- Fusion reagents are easy to find and inexpensive (for the deuterium) or easy to create (for the tritium).
Why doesn’t every atom fuse on earth? Because of the Coulomb force, the repulsive electric force that prevents them from doing so. This means that to fuse two atoms, it is necessary to overcome this repulsion. In this way, we must manage to make the atoms collide, at a very high speed, which causes them to fuse.
Several processes are developed to achieve artificial fusion, but two major ones stand out: Magnetic confinement fusion and Inertial confinement fusion. Magnetic confinement involves using magnetic fields to contain and control the plasma (that we’ll define a bit lower), over a long period. While inertial confinement involves using high-intensity lasers or particle beams to compress and heat a small pellet of fuel to extremely high temperatures and pressures, creating a plasma. Confined only by its own inertia, the plasma survives for only about one billionth of a second (one nanosecond).
The thing to remember is that each of these processes uses a different method of fuel heating and containment, but the expected result is the same: generating energy thanks to fusion. Even within these two categories different machines and techniques exist:
-Magnetic confinement: Tokamak, Stellarator, Magnetic mirrory, etc… -Inertial confinement: Indirect or direct drive, Ion Beams, Z machine…
Of all of these different technologies, the tokamak is the most well-developed and well-funded approach. Indeed the 3 main records relating to nuclear fusion are held by tokamaks:
- Highest temperature reached: 522 million degrees Celsius (1996), JT-60 (Japan)
- Longer plasma maintenance time: 17 minutes 36 seconds (December 2021) HL-2M (China)
- Greater energy and power of nuclear fusion: 59 MJ in 5 seconds (February 2022) JET (United Kingdom)
What are tokamaks, and how do they work?
Since tokamaks seem to be the devices that bring us closer to commercial fusion, let me explain to you how this complex device works.
A tokamak is a torus-shaped device that uses a powerful magnetic field to confine the hot plasma.
Important to note: plasma is a fundamental state of matter (one of the four). When a system goes into plasma form, the electrons are detached from the atom’s nuclei. Plasma is an ionized gas, it can be characterized as a kind of soup of electrons and ions. Another major point to take into consideration is that plasma is very sensitive to electric, magnetic, and electromagnetic fields.
Here’s what a tokamak looks like, impressive right?
What are the challenges to face to achieve nuclear fusion?
1 One of the major problems we encounter is that we have to heat the plasma at a very high temperature. On the sun, the plasma heating and confinement process is ensured by the high temperature, of course, but above all by the very great gravitational force of the sun (generated by its mass). It’s impossible to recreate this gravitational force on Earth, so we need to heat the plasma even more, to about 100,000,000 Kelvin, six times the temperature of the Sun’s core.
2Another problem we are facing is that plasma loses energy (heat), so sustaining a hot plasma requires that fusion reactions add enough energy to balance the energy losses. We have to keep the plasma hot enough and condensed enough for long enough.
3 We need a considerable quantity of energy to start the fusion reaction: to heat the plasma and to confine it. For the reaction to be profitable in terms of energy (having a higher output of energy than the energy input), the plasma reaction must be self-sustaining. But how can the plasma be self-sustaining?
Plasma and fusion reaction process in the tokamak
As I mentioned, the “classical” fusion reaction combines a deuteron (the nucleus of a deuterium atom) with a triton (the nucleus of a tritium atom). To make it short, that’s the reaction we want to take place in tokamaks because that’s the “easiest” to recreate on Earth. The two products of the reaction are an alpha particle (the nucleus of a helium atom) at an energy of 3.5 million electron volts (MeV) and a neutron at an energy of 14.1 MeV.
Since the neutron has no electric charge, it is not affected by the surrounding electromagnetic field (that we’ll address later), in that way, the neutron ends up in the surrounding material: the lithium blanket. Then two things happen:
- The heat generated by this collision between the neutron and the blanket can be converted into electricity with a classical process: steam-driven turbines.
- When the neutron collides with the lithium, a triton (and an alpha particle) is produced and can be used for the fusion reaction! This means that deuterium-tritium reactors use their waste (neutrons) to generate more fuel! This will cut costs because the tritium is the rarer (so the most expensive) element of this reaction to find and extract: the deuterium can be obtained with seawater and the lithium is easily found, so both are inexpensive.
The alpha particles are electrically charged, then, they are affected by the electromagnetic field. This means that, after their production, they are incorporated into the plasma, where they collide with the deuterons and tritons. In this way, alpha particles are “transferring” their energy to the plasma. This reaction can therefore increase the plasma energy (heat) and exceed the loss of energy, we call this self-sustaining, or “ignited” plasma.
How is plasma heated in tokamaks?
There are several techniques for heating the plasma, here they are:
- Ohmic heating: As we said, plasma is an ionized gas, which means that it is possible to induce an electrical current through it. Heat generation is permitted because of the resistance of the plasma (a similar process as in an electrical heater or a light bulb). This method should be complemented by another one. Indeed, as the temperature of the plasma increases its resistance decreases, so ohmic heating becomes less effective. (The maximum plasma temperature to reach in a tokamak with ohmic heating is 20–30 million degrees Celsius.)
- Magnetic compression: According to the ideal gas law (pV = nRT), a gas temperature can increase if the pressure undergone by the gas increases. It’s the same for plasma: the temperature of the plasma can be increased if it is compressed rapidly, by increasing the confining magnetic field. This can be easily done by moving the plasma into a region of higher magnetic field. Since plasma compression brings the ions closer together, the process has the additional benefit of facilitating the attainment of the required density for a fusion reactor.
- Neutral-beam injection: Neutral-beam injection involves the introduction of high energy (rapidly moving) atoms (neutral=unaffected by the magnetic field) into an ohmically heated, magnetically confined plasma within the tokamak. When the beam of neutral atoms is injected into the plasma, the neutral atoms are ionized by the plasma, forming a beam of ions. The beam then transfers its energy to the plasma through collisions, heating the plasma and increasing its temperature. The neutral-beam injection technique is typically used in conjunction with other plasma heating methods, such as radio-frequency heating, to achieve the high temperatures needed for fusion.
- Radio-frequency heating: We can generate high-frequency electromagnetic waves outside the torus with oscillators. A precise frequency and polarization should be calculated, if they are correct the waves can transfer their energy to the plasma particles, increasing the plasma heat. This works with microwaves as well.
Different heating methods can be used at different stages of plasma heating, and some methods may be more effective at certain stages than others. For example, neutral beam injection can be used to heat the plasma to relatively low temperatures, while radio-frequency waves can be used to sustain the plasma at higher temperatures over a longer period. Many tokamaks use a couple of different methods of heating to bring the plasma to the very high temperature required.
How is plasma confined?
Now that we know how to heat our plasma, how should this very high-temperature matter be confined?
The first thing to consider is that the plasma must not touch any surfaces of the torus, indeed these surfaces are much colder than the plasma, so it must never be in contact with them, otherwise, it would cool.
Something I didn’t mention earlier is that the plasma is contained in a vacuum chamber! So the first step to achieving fusion in a tokamak is to create a vacuum inside the torus to prevent the plasma from being cooled by interactions with air molecules.
Then, the gas is introduced into the torus and heated as explained earlier. But we should keep the plasma away from the torus’ surface. For that, we have electromagnetic fields! The hot plasma is kept away from the walls of the torus by magnetic coils that line the vessel. To confine the plasma, tokamaks have two fields that keep the plasma stable and in equilibrium, horizontally and vertically:
- The toroidal field: This is the main magnetic field that surrounds the plasma in a tokamak. It is created by electric currents flowing in the magnetic coils that encircle the plasma. The field lines of the toroidal field are shaped like a torus, with the plasma confined inside.
- The poloidal field is generated by a central solenoid, a magnet in the form of a coil carrying an electric current, confining the plasma particles. This one is represented as the “Primary coil” on the sectional representation of the tokamak.
To better understand, here’s a representation of those two fields:
The two field components result in a twisted magnetic field that confines the particles in the plasma. A third set of field coils generates an outer poloidal field that shapes and positions the plasma (represented as position control coils in the next figure).
Superconductivity in tokamaks
Today, tokamaks use superconducting magnets (electromagnets that are made from coils of superconducting wire).
Important to note: Superconductivity is a phenomenon that occurs in certain materials when they are cooled to very low temperatures (close to absolute zero). At this temperature, the electrons are able to move inside the material with almost no resistance. This results in the material being able to conduct electricity with almost 100% efficiency, which is why it’s called a superconductor.
These superconducting magnets are amazing for a couple of reasons:
- They create super-powerful electromagnetic fields
- They avoid the heating issue: when a material with electrical resistance is traversed by an electrical current, heat is generated, due to the resistance, and that heat must be evacuated.
- When electrical resistance exists, we need to continuously bring energy to cope with the energy loss. Without resistance, electrical consumption would drop to zero and, as an added advantage, no heat would be created in the magnets.
In the end
If all the previous conditions and characteristics mentioned that are:
- Qualitative and consistent plasma heating, through different methods.
- Qualitative magnetic fields keep the plasma in a shape in which it stays stable.
are met, then plasma particles can collide at very high speed and temperature, and, overcoming the Coulomb force, they fuse!
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