Vast amounts of money and brainpower in the effort to achieve sustained, controlled, nuclear fusion. Many designs have been proposed, and even tried, but tokamaks are comfortably the most popular. The name may be familiar, but not everyone knows what they are. 

What Is A Tokamak?

Stars heat matter to such high temperatures that electrons escape their atoms, creating plasma. A tokamak is a device designed to hold plasma so we can bend it to our will, which ultimately is to replicate the fusion that takes place at the core of stars.

On Earth, a plasma would normally interact with its surroundings, losing energy to them and cooling down, even if it didn’t melt the walls. To avoid this, tokamaks use powerful magnetic fields to hold the plasma in a torus, a shape most familiar to us from donuts (not the sort with jam centers). 

Many smaller tokamaks have been built so we can do basic research on the behavior of plasma. However, the major reason the world has spent tens of billions of dollars building larger and more powerful versions is the hope we can eventually get enough plasma inside a tokamak to fuse to produce useful amounts of energy. The sun achieves nuclear fusion through the immense force gravity applies at its core, but it takes a star’s worth of matter to do that. It is hoped tokamaks can allow us to do something similar without all that mass.

When the nuclei of small atoms fuse, the resulting nucleus is slightly lighter than the components that went into making it. The lost mass is released as energy. By the famous equation E=mc², a very small amount of mass becomes a great deal of energy. When four hydrogen atoms (or two deuterium atoms) fuse to become one helium atom, the mass loss is so tiny, the energy released is still small. However, if you can manage to fuse a steady supply of atoms, the energy released becomes immense. 

How Should Tokamaks Work?

Since plasma consists of atoms whose electrons have been stripped away, it is positively charged. The motion of an electrically charged material can be controlled through electrical or magnetic fields. Tokamaks exploit this to keep plasma within the donut shape without letting it touch anything and lose energy.

To achieve fusion, the temperature of plasma needs to be very high. Like a gas, plasma will expand if not contained, and the hotter it is, the more pressure is required to stop it escaping. That means the fields required to make Tokamak’s work have to be enormous.

These mighty fields are produced by wrapping the torus in conductive coils and putting vast amounts of current through them.

Once the field is sufficient to keep the plasma where it is needed, a pulse of energy is delivered, for example with lasers, ion beams or a burst of electrical current known as a Z-pinch. Atomic nuclei get squeezed together so hard that in some cases it overcome the repulsive forces between them, causing them to fuse.

At least some of the leftover energy is released as heat. In theory, this can be captured to heat a liquid to become gas to drive a turbine. Using steam to drive turbines is the dominant way of producing electricity today, but of course the process is a lot more difficult when the plasma is already hot enough to destroy anything it touches. A popular solution is to fuse deuterium and tritium nuclei together. Besides producing helium and heat, the reaction produces neutrons, which carry away a lot of the energy at great speed. Not being electrically charged, neutrons can escape the magnetic field, and heat surrounding materials.

What Have Tokamaks Done?

Tokamaks have been used to fuse nuclei for decades, but you might have noticed none of the electricity in your wires is made this way. That’s because enormous amounts of energy are required to heat and contain the plasma, and then induce the pulses that initiate the fusion process.

Currently, far more energy is supplied than produced. 

In 2022 a great deal of fuss was made about what was described as the first controlled fusion reaction with a positive energy gain, that is its energy output was greater than its input. The National Ignition Facility (NIF), where the work was done, was not a tokamak, but several tokamaks are hot on its tail. 

However, while the NIF claim was true, much of the coverage was very misleading. One acknowledged issue is that the reaction lasted more than a few seconds, but the problems were bigger than that.

The reaction’s output only exceeded input if you looked at the single stage where lasers put energy in and power came out. This ignored all the energy required to power the facility to get there, and the challenges of harvesting the energy produced in the fusion reaction. 

Consider the later part of the process. Coal and gas-fired power stations are really only about 33 percent efficient. A great deal of heat is produced when the fuel is burned, and while some of this is used to drive a turbine, a lot is wasted. Some are released as hot flue gasses or steam into the environment, some end up heating up the walls of the power station, and the turbine is not perfectly efficient at turning its rotational energy into electricity.

The challenges of capturing the heat produced in a tokamak or other fusion reactor will be even greater, so it’s almost certain the efficiency will be lower still. If two-thirds of the heat the reactor produces never gets turned into electricity, then we need to get three times as much heat out for the energy put in to make the reaction happen before its energy positive in practical terms.

So far, no tokamak has come close to that, and it’s unlikely one will for many years.

Even once positive in terms of useful energy, tokamaks will have a fair way to go before they’re widely used. Imagine if you put a gigawatt of electricity into a fusion power plant and got 1.1 GW out. You’re ahead, but you’re getting out less energy than most existing power stations produce, or a large wind farm. Considering the enormous cost of building a machine like that, you’re going to want a much better return.

Are Tokamaks The Future Of Energy?

There is a cliché that practical fusion power is twenty years away – and has been for twenty years. This is somewhat true, except that boosters have been promising fusion power within two decades for sixty years now, not twenty. Every now and then the promises get more ambitious, such as the time that Lockheed Martin promised a workable fusion reactor within ten years. That was ten years ago, and there is no sign of delivery.

That history means that only the very naïve would take predictions of imminent fusion success at face value. Nevertheless, practical nuclear fusion is something we will almost certainly achieve eventually, barring complete civilization collapse. Whether it will dominate our energy production is a much more doubtful proposition.

Fusion first came to be hailed as the future of energy in the 1950s when the major cost of producing energy was in the fuel. Most of the world’s electricity came from coal, which was dirty and dangerous to mine. As the best coal sites got used up, the cost would eventually rise further.

Nuclear power based on fissioning uranium (or thorium) meant much cheaper and more abundant fuel, but eventually this would run out as well, people reasoned, in addition to the problem of disposing of the waste. In nuclear fusion the fuel, available from sea water, would cost almost nothing. Once we worked out how to do it, how could fusion not be the path to limitless energy too cheap to meter?

Today, however, a rising share of our electricity comes from other sources where the fuel is free, in the form of sunlight and wind. That doesn’t mean the electricity cost is nothing, because the installation costs have to be repaid.

The first fusion reactors will cost tens of billions of dollars to build, and will have to compete with solar panels whose price has been falling for 50 years, and will probably be cheaper still by that point. Fusion will have one big advantage: that it can work 24/7. Still, the cost of building a working tokamak and the capacity to harness its power will probably be many times that of a similarly powerful solar farm.

Consequently, whether using tokamaks makes sense will come down to whether they can be built for less than the cost of solar panels and batteries (or other storage methods) combined. That’s assuming they keep their advantage over other fusion designs. That’s a question whose answer will probably vary by location. Close to the Arctic Circle, or on missions to the outer reaches of the Solar System, tokamaks will be hard to beat as a source of energy, but whether they will provide most of humanity’s needs, or remain a niche, is something we won’t know for a lot more than 20 years.