Poloidal field coils of the ASDEX tokamak.

In tokamaks, the magnetic cage has the shape of a doughnut and is created by the combination of the toroidal magnetic field and the poloidal field generated by the plasma current. Additional poloidal field coils — large rings encircling the torus outside the toroidal field coils — are used to shape and position the plasma and to maintain its equilibrium and stability. In ITER, six poloidal field coils range up to 24 metres in diameter. The four largest were too large to be transported, so a special winding facility was built on the ITER site for their manufacture.

Schematic of toroidal and poloidal field coils in a tokamak.