What is a Tokamak?

Since we have now established what nuclear fusion is, and its potential as an attractive source of energy, the next obvious question is: How do we create fusion in a laboratory? This is where tokamaks come in.

The key features of a tomakak are shown above.

In order for nuclear fusion to occur, the nuclei inside of the plasma must first be extremely hot, like in a star. For example, in the Alcator C-Mod tokamak we routinely create plasmas which reach temperatures of 90,000,000 degrees Celsius, about 5 times hotter than the center of the Sun. Unfortunately, no material on Earth can withstand these temperatures so in order to contain a plasma with such high temperatures, we have to be creative. One clever solution is to create a magnetic “bottle” using large magnet coils to capture the plasma and suspend it away from the container’s surfaces. The plasma follows along the magnetic field, suspended away from the walls. This complex combination of magnets used to confine the plasma and the chamber where the plasma is held is known as a tokamak. Tokamaks have a toroidal shape (i.e. they are shaped like a donut) so they have no open ends for plasma to escape. A picture demonstrating key features of a tokamak is shown to the upper right. Tokamaks create and contain the hottest materials in the solar system.

The Alcator C-Mod Tokamak

The Alcator C-Mod tokamak, located at the MIT Plasma Science and Fusion Center in Cambridge, MA, has the highest magnetic field strength of any fusion device in the world: reaching fields of approximately 100,000 times the Earth’s magnetic field. This allows it contain the densest plasma at the highest pressures anywhere in the world. On a daily basis the Alcator C-Mod tokamak confines plasmas up to 90,000,000 degrees Celsius and creates up to 100 trillion fusion reactions per second.

Tokamaks around the world

There are currently over a dozen major tokamaks in labs around the world and many smaller ones in universities. Each tokamak is different in order to study different parts of plasma physics and fusion energy. In addition to Alcator C-Mod, the US currently operates two other major tokamaks: DIII-D (pronounced “Dee 3 Dee”) in San Diego and NSTX at the Princeton Plasma Physics Laboratory. Each of these tokamaks is descended from a long line of US tokamaks, with higher performance from each generation. Together these three tokamaks represent a substantial investment by the US in the future of fusion energy.

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