#148 What is a tokamak?

I learned this today. A tokamak is a device that uses a strong magnetic field to keep a gas plasma in a donut shape in order to achieve nuclear fusion.

Tokamaks were invented in Russia in the 1960s and the word comes from Russian. It means “toroidal chamber with magnetic coils”, which is тороидальная камера с магнитными катушками in Russian.

Tokamaks are necessary if we want to have nuclear fusion here on Earth. Nuclear fusion is the process by which the sun makes energy. When four hydrogen atoms collide and stick together, they become a helium atom. Four hydrogen atoms weigh a tiny bit more than a helium atom and this tiny bit of mass becomes energy. This process can happen in the sun because it requires an enormous amount of energy to get the hydrogen atoms to stick together because it is necessary to overcome the repulsion between atoms. Stars can do this because the pressure at their centers caused by their enormous gravities creates stupendous heat and energy. The energy released from the nuclear fusion reaction adds more heat and energy to the mix and the reaction keeps going.

On Earth, in order to start a nuclear fusion reaction, we need to bring the atoms to a ridiculous temperature. This is because we don’t have the gravity on Earth that the sun has. The atoms have to be heated up to a minimum of 150 million degrees Celsius. And that is not easy to do. Also, something at that temperature would burn through anything it came into contact with.

The first attempts at nuclear fusion happened in 1934. Scientists used a particle accelerator to fire deuterium nuclei into a metal foil container that held deuterium atoms. They managed to achieve nuclear fusion, but they could not control the reaction and energy from this reaction was lost to the environment. If the energy cannot be recycled into the fusion, there is no way to keep it going.

Scientists realized that they would need to heat the atoms up to incredible temperatures to give them so much energy that they are constantly colliding at high speed. Enrico Fermi estimated that the temperature would be 50,000,000 K, but it turned out to be more than three times that.

One way of providing the heat was to use a regular nuclear bomb to start the fusion reaction. This is what happens in a fusion bomb. Shortly after the atomic bombs were dropped on Japan, Enrico Fermi started lecturing about the possibility of a hydrogen bomb. It would use a regular nuclear fission reaction to create enormous pressure and heat in order to force hydrogen atoms to undergo nuclear fusion.

A tokamak has nothing to do with using nuclear fusion as a weapon. The goal for starting a nuclear fusion reaction on Earth is for the energy it would provide. If a nuclear fusion reaction could be sustained, it would provide unlimited energy. There would be no carbon emissions, there is unlimited fuel, they are incredibly efficient, there is little radiation, and in the event of an accident they would just switch off.

A tokamak is at the center of a nuclear fusion reactor. To start a nuclear fusion reaction in a tokamak, the air is sucked out of the chamber to make a vacuum. Then the super powerful magnets are switched on. These are not regular magnets. For example, the ITER project in the South of France uses 10,000 tons of combined magnets. ITER is a combined effort by 36 different countries to make the world’s largest tokamak. These magnets are made from niobium-tin and they are supercooled to about 4 degrees Celsius above absolute zero with supercritical helium. At this temperature they become superconductors which means they can conduct electricity without any resistance.

Deuterium and tritium are added into the tokamak chamber and a strong electric current is introduced, heating the gas until it becomes a plasma, which is an extremely hot and electrically charged gas. The nuclei start to combine into a helium nucleus and a neutron, and the left-over mass becomes energy. This energy can be conducted out of the tokamak.

To keep the plasma moving and colliding, the incredibly strong magnets are used. The center of the tokamak is a donut shape (called a torus) and the first set of magnets send the plasma the long way around this shape. A second set of magnets send the plasma the opposite way, making a twisted magnetic field that keeps all of the particles in the plasma. A third set of magnets makes an outer field that shapes and positions the plasma. Using these three magnetic fields, it is possible to keep the nuclei colliding. The goal is to make a reaction that makes enough energy to sustain itself.

The technology to have sustained nuclear fusion has been a dream since it was first achieved in the 1930s. However, recent advances in the ability of magnetic materials and supercooling have meant that it is no longer a dream. In June of 2021, China’s Experimental Advanced Superconducting Tokamak reactor managed to sustain nuclear fusion for 101 seconds! And the ITER reactor, which is scheduled to be active this fall, has even more technological advances. I always thought nuclear fusion would not be possible in my lifetime, but it is looking like it might be possible in the next few years. All thanks to the tokamak. And this is what I learned today.

Photograph By Rswilcox – Own work, https://commons.wikimedia.org/w/index.php?curid=73919081

Sources:

https://www.iter.org/mach/Tokamak

https://en.wikipedia.org/wiki/Tokamak

https://en.wikipedia.org/wiki/Fusion_power

https://en.wikipedia.org/wiki/Plasma_(physics)

https://www.energy.gov/science/doe-explainstokamaks

https://www.energy.gov/science/doe-explainsplasma-confinement

https://en.wikipedia.org/wiki/Thermonuclear_weapon