What do refrigerators, levitating Maglev trains, earphones, MRI machines, and metal-scrap separating machines have in common? Why, magnets! Since the supposed discovery of lodestone (a primitive magnet) by the Greek shepherd Magnes 2500 years ago, humans have come a long way in using magnets. Go back in time a couple of hundred years and people probably wouldn’t believe that magnets could be used for much more than navigating the seven seas. However, while the use of magnets has captured the public imagination in most spheres, one of the less cited uses of magnets is their use in generating fusion energy. Let’s take a look, shall we?
A Primer on Fusion
Back in the 1930’s, a guy by the name of Hans Bethe had a burning question that science couldn’t answer at the time – what powers a gargantuan source of energy like the Sun? The answer is nuclear fusion. 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). The difference in mass between the reactants and products is manifested as either the release or absorption of energy. This difference in mass arises due to the difference in atomic “binding energy” between the atomic nuclei before and after the reaction.
Fusion reactions are very rigid about the physical conditions at which they proceed. These conditions include a very high reaction temperature around 100 million Kelvin (6 times hotter than the Sun’s core), a controlled reaction environment (to prevent the spread of toxic and harmful radiations released), and a high pressure to squeeze the atoms together so that they are within mere femtometers of each other. Large hot bodies like the Sun can achieve these conditions with the help of their large mass and the force of gravity compressing this mass in the core. Furthermore, the Sun uses its mass and the force of gravity to squeeze hydrogen atoms (our fusion fuel) together in its core. Here on Earth, we squeeze hydrogen atoms together by using powerful lasers, or ion beams. Permanent magnets are also handy in achieving close to ideal conditions. When heated to fusion temperatures, the electrons in atoms disassociate, resulting in a fluid of nuclei and electrons known as a plasma, which is electrically conductive. Hence, the plasma can be influenced by electrical and magnetic fields, which brings in the possibility of using both permanent magnets and electromagnets. Let’s now delve further down this rabbit hole of the uses of magnetism in fusion reactors, shall we?
Fig. 1: Electromagnets and Permanent magnets
Fusion Reactors and Magnets
Magnets are primarily used in confining the plasma generated inside a fusion reactor. Confinement refers to prevention of leakage of the plasma out of the reactor setting and preventing it from touching the physical walls of the setup. Confinement is important because the very hot temperature of the plasma can melt any Earthly material and the free electrons are highly reactive (can cause corrosion and hamper living cellular processes), which makes plasma a tremendous safety hazard. The two most common confinement designs are the Tokamak and the Stellarator.
A Tokamak (Russian acronym that stands for “toroidal chamber with magnetic coils“) is a device that uses a powerful permanent magnetic field to confine the hot plasma in the shape of a torus. The charged particles of the plasma can be influenced to move in such a pattern that it confines the hot plasma away from the vessel walls. The process begins with the air and impurities being evacuated first from the vacuum chamber. Next, the permanent magnet systems that help to confine and control the plasma are charged up, and the gaseous fuel is introduced. As a powerful electrical current is run through the vessel, the gas breaks down electrically, becomes ionized (electrons are stripped from the nuclei), and forms a plasma. As the plasma particles become energized and collide, they also begin to heat up. Auxiliary heating methods help to bring the plasma to fusion temperatures (between 150 and 300 million °C). Particles “energized” to such a degree can overcome their natural electromagnetic repulsion on collision to fuse, releasing vast amounts of energy.
Fig. 2A: Assembly of a Tokamak
Fig. 2B: Working of a Tokamak
Stellarators consist of a set of complex twisted coils that spiral like stripes on a candy cane to produce electromagnetic fields that shape and control the plasma that fuels fusion reactions. The idea is, by the time a particle flows from the outside of one of the curved areas through the straight area and into the other curved section, it will be inside the center. In short, the upward drift on one side is counteracted by the downward drift on the other. Toroidal field coils can also be used to control the magnetic surface characteristics. The basic fusion reactions and, thus, the process of energy generation that occurs inside the Stellarator is the same as in the Tokamak. The sole difference is that the Stellarator makes use of a helical coil and a pair of poloidal field coils to provide a vertical field to confine the plasma, while the Tokamak utilizes permanent magnets which produce a purely circular magnetic field for the same purpose.
Fig. 3A: Assembly of a Stellarator
Fig. 3B: Complex field arrangement inside a Stellarator
As the gifs above show, the setup of a Stellarator’s coils are quite complex. Researchers believe that refrigerator-like permanent magnets could produce the required fields without additional components, allowing for a simpler setup as in the Tokamak. Moreover, Stellarators based on permanent magnets rather than electromagnets do not run the risk of damaging disruptions that more widely used Tokamak fusion devices face.
Apart from magnets, such reactors could also have used electrostatic fields or lasers for confinement. However, these methods bring their own set of challenges. Laser fusion which is known as ‘inertial laser confinement fusion,’ is a method of initiating nuclear fusion reactions through heating and compressing pellet-shaped fuel targets. However, the major challenge here is that even with the most advanced lasers today is that they are relatively inefficient at converting electrical energy into beam energy. Besides, not all of the laser energy ends up being delivered to the fusion target due to scattering or reflection off surfaces. Inertial electrostatic confinement, or IEC, is a class of fusion power devices that use electric fields to confine the plasma rather than the more common approach using magnetic fields found in magnetic fusion energy (MFE) designs. But then again, such fields could cause thermalisation of the radioactive ions and if such a system is heated adequately, we would see a large loss in net power produced due to X-Ray emission. The electrostatic process of confinement also leads to the formation of an unstructured Gaussian Surface which increases the surface area through which energy can be lost and also increases the raw reactants required for fusion.
Now that we have settled on using magnets, let’s look at why permanent magnets are the better bet. Permanent magnets are always ‘on,’ in sharp contrast to the standard electromagnetic coils that present day Stellarators and Tokamaks use, which require a constant electric current to be supplied to be operational. Permanent magnets are also lower cost than hand-crafted electromagnets and can be easily repositioned to create a variety of field patterns. Permanent magnets are fixed solids they are simpler to maintain at high temperatures. Lastly, scientists have shown that permanent magnets have a lower environmental impact, which is of prime importance today.
These almost magic materials do have disadvantages. The ability to turn magnets ‘OFF’ is important for longevity. Moreover, permanent magnets are also limited in the field strengths generated. Presently, researchers are working on developing stronger magnets and using the current magnets for tabletop fusion applications (Back To The Future, anyone?).
Fusion: Our Energy Messiah?
By the end of the century, demand for energy will have tripled under the combined pressure of population growth, increased urbanization and an explosion in the number of people connected to the grid in developing countries. This is where fusion comes in to save the day. The primary advantage is that fusion fuels are nearly inexhaustible! Deuterium can be distilled from all forms of water, while tritium will be produced during the fusion reaction as fusion neutrons interact with lithium. (Terrestrial reserves of lithium would permit the operation of fusion power plants for more than 1,000 years, while sea-based reserves of lithium would fulfil needs for millions of years.)
However, transitioning to 100% fusion energy does come with a hefty price. Early 2008 data predicted that the lump sum cost of these reactors would be between $6 billion and $9 billion. By the year 2035, this price could easily cross $22 billion and even more, raising further doubts on their viability. For reference, this cost is much higher than that of other forms of energy production, such as thermal or hydroelectric. It is important to realise however, that these estimates are based on the assumption that fusion energy can’t be mass produced. Once the process of fusion energy generation matures, economies of scale will inevitably bring the costs down tremendously. Plastics made a similar jump in cost in the last 50 years. Scientists are also addressing the radioactive waste stream (forget human hands, this can damage shielded workshop cranes!) generated during operation and after decommissioning. Powerful magnets can be used to securely contain the final residue. Hence, permanent magnets nip the waste problem in the bud by reducing the amount of waste generated.
All in all, where humankind is very close to harnessing the marvels of fusion energy. It could prove to be the solution to nearly limitless energy generation and could reduce our dependence on other fuel sources. However, it is equally important to realise that fusion energy is not a one pill cures all solution. Our past mistakes have taught us that it is best to avoid overreliance on a single fuel, since it avoids the scampering around when an energy source no longer becomes viable. This is what we are seeing today when fossil fuels are running out. Fusion fuels also lead to reduced carbon emissions and nearly no hazardous radioactive fission waste.
The ingenious designs of the Tokamak and the Stellarator raise our hopes for fusion energy. Magnets could help solve many of the challenges these designs face currently. In the end, with or without magnets, fusion energy is on the horizon but it is upto our generation of scientists to make it a reality!