Tokamak ASDEX poloidal coils.

In tokamaks, the magnetic cage has the shape of a donut and is created by the combination of magnetic fields generated by toroidal coils and current flowing through the plasma column. For greater stability, the vertical magnetic field is needed. This is provided by poloidal coils — large rings encircling the whole torus, including its toroidal coils. A set of several poloidal coils with different diameters pushes expanding plasma back to the centre of the torus, providing better stability of the plasma column and is usually used for positioning and shaping the plasma. They are sometimes quite large structures — in ITER, the largest tokamak in the world now under construction, the biggest poloidal coil has a diameter of 24 metres and is so big that it cannot be transported, so a special winding facility was built on-site for its construction.