Subscribe options

Select your newsletters:


Please enter your email address:

@

News & Media

Latest ITER Newsline

  • Neutral beam test facility | First ITER test bed enters operation

    For all those who had contributed to designing and building the world's largest negative ion source, it was a deeply symbolic moment. ITER Director-General Bern [...]

    Read more

  • ITER Robots: pre-teens can too!

    Made-from-scratch movers and carriers were again on display near ITER, as the younger set took up the ITER Robots challenge. From two participating school [...]

    Read more

  • Supporting crown | A midnight pour

    It is close to midnight in the brightly lit basement of the ITER bioshield and, tonight, the first plot of the tokamak 'crown' is to be poured. The operation is [...]

    Read more

  • Prototype | The hottest stuff in ITER

    The heat flux sustained by the targets of the ITER divertor will be higher still—by ten times—than that of a space vessel re-entering Earth's atmosphere. Meticu [...]

    Read more

  • Gyrotrons | In Russia, that makes two

    In mid-May, factory acceptance tests were successfully carried out on the second gyrotron of the Russian procurement program by specialists at the Institute of [...]

    Read more

Of Interest

See archived articles

Fusion machines

Searching for the perfect shape

Kirsten Haupt

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