Toroidal (blue) and poloidal (red) directions on a torus.

An ideal magnetic cage for hot plasma can be represented by an infinitely long solenoid. For practical use, however, the magnetic configuration must be finite, so the solenoid is bent into the shape of a torus. Toroidal field coils arranged around the torus produce the toroidal component of the magnetic field used for plasma confinement.

Toroidal Coils

Each tokamak has its own configuration of toroidal field coils, with different numbers, sizes and shapes. A larger number of coils generally produces a more uniform toroidal magnetic field, but the number of coils is limited by available space. Their inner sections must pass through the central column of the torus, where space is particularly restricted. In spherical tokamaks, this central column is made as narrow as practical. Space must also be left between the coils to provide access to the vacuum vessel for diagnostics, maintenance, pumping and fuelling systems.

Toroidal Coils of Current Tokamaks

The largest operating tokamak, JT-60SA, uses 18 toroidal field coils, while the former JET tokamak had 32. Large modern tokamaks commonly use D-shaped toroidal field coils, which provide space for the plasma and help accommodate the mechanical loads produced by the strong magnetic field.

ITER, the world’s largest tokamak under construction, will use 18 D-shaped toroidal field coils, each about 17 metres high and 9 metres wide. The coils are designed for a maximum magnetic field of 11.8 tesla and together will store about 41 gigajoules of magnetic energy. Many experimental tokamaks operate with toroidal magnetic fields of a few tesla.

2 pictures

2 pictures