Understanding the hype: fusion 101
Nuclear fusion is cool or hot — pick your favourite adjective. This article is part of our fusion series. Read the main fusion article here. Keen to know more about the commercialisation aspect of fusion? Get all the details here.
By Iris ten Have, Eshel Lipman & Torben Schreiter
There are many reasons why people get excited about nuclear fusion. To us, it boils down to four:
- Carbon-free. Fusion doesn’t emit CO2; the only by-products of fusion reactions are small amounts of helium.
- Baseload energy qualities. Fusion could provide energy 24/7 (unlike wind and solar, which are both intermittent and would need storage).
- Fusion fuel is abundant and accessible. Fusion could cover the annual electricity demand of one German household (±3000 kWh) with the deuterium found in 45 litres of water and the lithium from an old laptop battery. In practice, it’s a little bit more complicated, but we’ll get to that later in the article.
- Energy density. Fusion could generate nearly four million times more energy per kilogram of fuel than burning oil or coal.
When two become one: what fusion is and the three parameters that define energy gain
So what is fusion? Let’s start with a simple explanation as most of us aren’t experts in plasma physics. Fusion is the process of combining two atoms into one while releasing a great amount of energy. Deuterium and tritium, essentially heavier forms of hydrogen, are the most common fusion fuels. While hydrogen consists of a positively charged proton surrounded by a negatively charged electron, deuterium’s core contains one additional neutron (neutrally charged particle) and tritium’s core has two neutrons. When these two fuse, helium and a neutron are formed and a huge amount of energy is released.
Why does fusion release so much energy? To understand this better, we should look at Albert Einstein’s famous formula E=mc^2. E stands for energy, m stands for mass, and c is a constant that indicates the speed of light (3 x 10^8 metres per second). Einstein’s equation reveals that energy and mass are related to each other. When the speed of light squared (c^2) is multiplied by the mass of a system, one obtains the energy of that system. This reveals that a tiny amount of mass can be converted into a huge amount of energy.
The nuclear binding energy allows this huge amount of energy to be released, which is related to a concept called “mass defect”. This concept tells us that the mass of an atom’s core (nucleus) is less than if the particles that the core is made of (nucleons) were separate. This might sound a bit abstract initially, but in relation to fusion, it effectively means that deuterium and tritium together are heavier than helium and a neutron together, and the mass difference multiplied by c^2 ( the speed of light) gives the nuclear binding energy which is the amount of energy released. Check out the illustration below:
Although the concept sounds straightforward, enabling nuclear fusion reactions in practice requires quite some effort. Plasma physicists do this through a combination of three parameters: density (n), temperature (T) and confinement time (τE). Generally, as a benchmark of progress, the triple product value (nTτE ) is used — a scientific term that represents the combined value of the plasma density, plasma confinement time, and temperature. A fusion reactor must reach a critical fuel particle density, confinement time, and plasma temperatures (Lawson criteria) to achieve a net positive energy output.
At a certain value of the triple product, called ignition, the reaction becomes self-sustaining: the heat generated by the reaction is enough to keep the plasma hot so that the external heating systems can be turned off. Let’s take a closer look at the essential parameters for fusion.
1. Temperature (T)
The higher the temperature, the more energetic the particles become. At high enough temperatures, particle energies can overcome electrostatic repulsion and get close enough to each other to fuse. This is known as thermonuclear ignition, and pushes the material into a state called plasma.
Plasma is the fourth state of matter in which electrons are not bound to atoms. Instead, they exist free in the presence of an electric field. This results in a highly electrically conductive gas that can be used to confine and control fusion reactions. To jumpstart any nuclear fusion reaction, the fusion fuel has to be heated using external sources.
2. Density (n)
Density refers to the number of particles in a given volume. The more particles there are in a given volume, the greater the chance they will collide and fuse. For fusion to occur, a certain critical mass must be reached. This is known as the Chandrasekhar limit.
In the next part, we will share several ways of increasing the density of the fusion fuel. In most cases, density is increased by either strong magnetic fields or through high pressure induced by powerful lasers.
3. Confinement time (τE)
Confinement time or energy confinement time indicates the rate at which the confined plasma loses energy to its environment. The confinement parameter is particularly important because it defines the time during which the plasma is self-sustained at a temperature above the critical ignition temperature. To yield more energy from the fusion than has been invested in heating the plasma, the plasma must be sustained at this temperature for a certain minimum time.
The three parameters above will help us get to the overarching holy grail in fusion: getting more energy out than we put in.
While obtaining net energy from nuclear fusion is mostly driven by physics considerations, building an actual reactor is mainly engineering. Currently, fusion devices are large in size. For example, during our fusion deep dive, we visited Wendelstein 7-X, which is 3.5 m high and has a diameter of 11 m. Certainly impressive, but not an in-your-own-backyard type of device. Luckily, the device size is driven by technology and engineering considerations, which makes the development of smaller devices eventually feasible. That said, let’s have a look at promising designs for fusion devices in the next section.
Harnessing the power of the stars: four mainstream approaches to creating confinement in fusion
So why does nuclear fusion occur spontaneously in the sun, but not on Earth? The sun is roughly 330,000 times as big as the Earth and thus a lot heavier. This results in very strong gravitational forces that naturally induce fusion. As the Earth is a lot lighter than the sun, we don’t have these gravitational forces. Therefore, to compensate for that lack of gravity when creating self-sustaining nuclear fusion on Earth, we need extremely high temperatures (>100 million degrees celsius), a long confinement time, and a certain density in order to increase the probability of nuclear fusion reactions to happen. In fact, the temperatures needed on Earth to create conditions for nuclear fusion need to be about 7 times higher than those at the centre of our sun.
How do we create these conditions artificially?
Looking at the sun, the first thing you may be thinking of is heating. Although external heating systems are certainly required to kick-start nuclear fusion reactions, plasma density and confinement time are more challenging to get right than heating. Therefore, we will focus mostly on methods to create high plasma density by compression and methods related to confinement time. The four most common approaches to facilitate nuclear fusion are tokamak, stellarator, Z-pinch, and lasers. Let’s take a closer look at each of them.
Tokamak
The tokamak is the most common type of magnetic confinement device. It uses a toroidal (doughnut-shaped) chamber and relies on strong magnetic fields to confine the plasma. In addition, the loop shape has traditionally been designed to reduce the amount of energetic particles (i.e. “alpha particles” or “helium nuclei”) escaping the confinement, which occurs in linear reactors. The loop shape keeps the plasma current continuous.
The most famous tokamak today is likely the International Thermonuclear Experimental Reactor (ITER). Currently under construction in France, this is the largest tokamak device to date and is intended to demonstrate the feasibility of fusion energy by roughly the end of the decade.
Tokamaks, in theory, are relatively simple devices and have been shown to work well in setting the basic stage and approaching ignition. They are also adjustable and modifiable in both size and volume. This eases the complexity of reaching very high temperatures and provides an approach to address the difficulty of reaching strong magnetic fields that force the use of expensive and complex superconductors. However, a major design problem is that controlling turbulence inside the plasma is difficult because alpha particles tend to want to “leave” the reactor fast, which causes significant efficiency losses.
Notable companies: Commonwealth Fusion Systems and Tokamak Energy.
Stellarator
The stellarator is another type of magnetic confinement device. It uses a similar chamber geometry but with more complex magnets arranged in such a way — helically — that they cancel out some of the unwanted forces acting on the plasma. This makes them more difficult and expensive to build, but they have several potential advantages over tokamaks, including better stability, reduced undesired turbulence and improved performance at high temperatures. Wendelstein 7-X, located in Greifswald, Germany, is the world’s largest fusion device of the stellarator type.
While Wendelstein is the largest fusion device, it is actually lacking a lithium blanket, which would be required to harvest energy. It is designed for research purposes only. To generate energy, the entire device would have to be completely redesigned as the radius of the stellarator would need to increase by ~1 m to fit the lithium blanket. Consequently, new helical coils would have to be designed and built, because they need to be positioned outside of the blanket. This effectively means that Wendelstein is — by design — incapable of generating any energy.
Stellarators are similar to tokamaks in the sense that they are both designed to support continuous processes. The plasma flow constantly generates neutrons, which are eventually captured by a lithium blanket that lines the sides of the reactor vessel. All existing prototype fusion reactors (tokamaks and stellarators) were built without lithium blankets to simplify setup. Therefore, it is impossible to actually convert energy into electricity with these prototypes. With lithium blankets in place, some of the heat generated will be used to create electricity, and this design additionally allows tritium to form and enables constant replenishment of the fusion fuel. There’s one catch though: only the less common isotope of lithium, Li-6, is suitable for both heat transfer and tritium production. Naturally occurring lithium mostly consists of Li-7, unconducive to the reaction, and contains just 2–8% Li-6. Given the fact that an effective lithium blanket will have to contain at least 40% Li-6, this could lead to supply chain issues.
Notable companies: Princeton Stellarators and Type One Energy.
Z-Pinch
Z-Pinches use a different approach to create magnetic confinement. By using electrical currents flowing through the plasma itself, the conducting plasma is consequently generating strong magnetic fields. These fields create huge forces, pushing the plasma further inside towards the centre of the container where it is held. This compressing process is described as a “pinch”. Theoretically, this pinch can create conditions that qualify as confinement. If the fuel pellet is located in the right position, this force has the potential to ignite fusion. However, the moment the pinch occurs, the magnetic fields change and eventually collapse, resulting in a very short confinement time. The challenge with short confinement times is that the system needs to reach even higher temperatures and densities (when we operate at a lower τE, we need higher values for n & T) to keep a sufficiently high triple product and the fusion reaction alive.
Notable company: Zap Energy.
Lasers
Lasers can create fusion conditions by using their high-energy beam to heat and compress a small pellet of fuel. An example of this is NIF’s recent accomplishment. Laser-based fusion reactors essentially hold the fuel in place while it’s getting blasted to achieve fusion.
What sets laser-based confinement apart from tokamaks or stellarators is mostly the way the fuel is being distributed. While in magnetic confinement the fuel will be spread all around the chamber, a laser-based reactor requires the production of special deuterium-tritium fuel pellets, and the insertion of each pellet into the specific convergence point of all of the lasers in the ignition chamber.
Notable companies: General Fusion, Fuse, and Marvel Fusion
Other approaches (and sub-approaches) are based on modified stellarators or other systems using both inertial and magnetic confinement, such as field-reversed configurations.
To sum it all up below is a market map of selected academic institutions and companies working on nuclear fusion. Wanna find out more about the commercialisation aspect of fusion? Get all the details here.
Many thanks to Dr. Angie Qarry for helping us understand the science and technology of hot fusion.
Continue to read our fusion deep dive here, where we go into the commercialisation aspect of fusion, or go back to the main article of the series.
