Enable Recite

Subscribe options

Select your newsletters:

Please enter your email address:

@

Your email address will only be used for the purpose of sending you the ITER Organization publication(s) that you have requested. ITER Organization will not transfer your email address or other personal data to any other party or use it for commercial purposes.

If you change your mind, you can easily unsubscribe by clicking the unsubscribe option at the bottom of an email you've received from ITER Organization.

For more information, see our Privacy policy.

News & Media

Latest ITER Newsline

  • Remembering Bernard Bigot, ITER Director-General 2015-2022

    On the ITER site, the machinery of construction was humming just like on any weekday. Workers were concentrating on their tasks, laying rebar for new buildings [...]

    Read more

  • Tokamak assembly | Preparing for the Big Lift

    The distance was short but the challenge daunting: on Thursday last week, the first section of the plasma chamber was lifted 50 centimetres above its suppor [...]

    Read more

  • Image of the week | 13th toroidal field coil arrives from Europe

    The toroidal field coil procurement effort has been one of the longest of the ITER program, initiated by Procurement Arrangements signed in 2007 and 2008. Manuf [...]

    Read more

  • Diagnostics | Final Procurement Arrangement signed

    ITER Diagnostics reached an important milestone in December 2021 when it concluded the last Procurement Arrangement of the diagnostics program. After signing a [...]

    Read more

  • On site | A quick visit to the Control Building

    Work is progressing on the ITER Control Building, ergonomically designed for the 60 to 80 operators, engineers and researchers who will call it home.  [...]

    Read more

Of Interest

See archived entries

Fusion machines

Searching for the perfect shape

The perfect magnetic trap may not exist, but the torus-shaped tokamak is currently the fusion device with the best performance on record so far. Source: EFDA-JET (now EUROfusion) (Click to view larger version...)
The perfect magnetic trap may not exist, but the torus-shaped tokamak is currently the fusion device with the best performance on record so far. Source: EFDA-JET (now EUROfusion)
The perfect magnetic trap doesn't exist. Over time plasma physicists have experimented with different types of cylinders, magnetic mirrors and circular or helical shapes to optimize control of the plasma. While R&D continues on many fusion energy configurations, the torus-shaped tokamak has yet to be dethroned as the highest performing fusion device.

What is the objective of a magnetic fusion trap? Fusion plasmas must remain in suspension in order to avoid contact between the superheated particles and the material vessel. As plasmas consist of electrically charged particles—positive ions and negative electrons—they can be controlled and confined by magnetic forces. ITER's magnetic "cage" will be created by superconducting coils shaping and controlling the plasma, as well as by electrical currents circulating within.

The first magnetic traps were open-ended cylinders. (Click to view larger version...)
The first magnetic traps were open-ended cylinders.
In the early days of plasma research, physicists experimented with cylindrical systems—devices with coils around a tube that created linear magnetic fields running parallel to the vessel body. But the "holes" in the magnetic trap—the cylinder's open ends—resulted in high losses of energy as the plasma particles escaped.

Source: WikiHelper2134 (Click to view larger version...)
Source: WikiHelper2134
Magnetic mirrors at the two openings of the device, essentially reflecting particles back into the cylinder, were an early attempt to solve the problem. Still, there were substantial losses of energy, despite the mirror trap.

The next solution came in the form of a closed system in which the magnetic field lines turn in on themselves—like a snake biting its tail—allowing the particles to spin indefinitely. The stellarator, with its complex geometry of twisted coils, was the first device to apply this shape, but using a complicated physical configuration that makes stellarators extremely challenging to build.

The complex geometry of the stellarator. Source: Max-Planck-Institut für Plasmaphysik (Click to view larger version...)
The complex geometry of the stellarator. Source: Max-Planck-Institut für Plasmaphysik
The torus-shaped tokamak, invented in Russia in the 1950s, also enables magnetic field lines that close to form a ring, but its smooth and symmetrical structure is much easier to build than the stellarator. However, there were early difficulties also with the tokamak design when experiments showed that electrically charged particles—while moving within the torus along magnetic field lines—would eventually drift off vertically, hit the walls and be lost.

This problem was resolved by inducing an electrical current inside the plasma, creating an additional magnetic field perpendicular to the current. As a result, the particles move in a three-dimensional curve, very much like a helix, and remain within the torus.

Today, the tokamak design rules supreme in the world of fusion. While innovators continue to experiment with a variety of devices, fuels, and approaches, the hydrogen-fuelled tokamak fusion reactor remains the device with the best performance on record so far.


return to the latest published articles