1 Tokamak basics
A tokamak is a doughnut-shaped vessel with armoured walls that can confine a hot plasma under high vacuum. It does this using a combination of magnetic fields to guide electrically charged particles: "toroidal" (blue arrow) and "poloidal" (green arrows) fields create a total field that twists around the torus in a gentle helix (purple). The poloidal field is generated by a toroidal current in the plasma, which is produced by a transformer that induces a current in the toroidal direction. With JET, this current has been as high as 7 MA and it will reach 15 MA in ITER. The toroidal magnetic field is provided by external, poloidally wound coils and is typically 10 times stronger than the poloidal component (in ITER a collection of 18 superconducting coils, each weighing a massive 300 tonnes, will generate a toroidal field of 5.3 T).
EFDA-JET