In a tokamak, plasma particles are confined and shaped by magnetic field lines that combine to act like an invisible bottle. Pictured, the spherical tokamak MAST at the Culham Centre for Fusion Energy (UK), where over 30,000 man-made ''stars'' have been created. Photo: CCFE
Physicists have been exploring the properties of plasmas within tokamak devices since the 1960s. The doughnut-shaped torus of the tokamak represented a major break-through in plasma science at the time, offering the conditions for temperature levels and plasma confinement times that had never before been reached.
The ITER Tokamak chamber will be twice as large as any previous tokamak, with a plasma volume of 840 cubic metres. Left to itself, the plasma would occupy all of the space in the chamber (1,400 m³), however no material could withstand contact with the extreme-temperature plasma. Scientists are able to contain or "confine" the plasma away from the walls by exploiting its properties.
Plasmas consist of charged particles—positive nuclei and negative electrons—that can be shaped and confined by magnetic forces. Like iron filings in the presence of a magnet, particles in the plasma will follow magnetic field lines. The magnetic field acts as a recipient that is not affected by heat like an ordinary solid container.
In ITER, different types of magnetic fields will work in subtle combination to shape the plasma into the form of a ring, or torus, and isolate the very hot plasma from the relatively cold vessel walls in order to retain the energy for as long as possible. The vacuum vessel is the first safety confinement barrier and will not be in contact with the plasma.
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