Magnetic confinement
Holding a hot plasma away from the vessel walls in a cage of magnetic field, using the fact that charged particles spiral along magnetic field lines.
No wall material can survive contact with a 100-million-degree plasma, so the plasma is held in mid-air by a magnetic field. Charged particles move freely along field lines but only with difficulty across them. The basic shape is therefore a closed, doughnut-shaped set of field lines; the tokamak and the stellarator are the two main forms.
Confinement quality is measured by how long the plasma keeps its heat — the energy confinement time. The fuel is far thinner than air, and the strategy is to hold it for a long time, from seconds up to continuously[1]. That is the opposite of inertial confinement, which crushes the fuel hard for an instant.
Performance depends strongly on field strength, so superconducting magnets that reach higher fields are the key to smaller, cheaper machines.
Related terms
- TokamakA doughnut-shaped machine that confines plasma by combining the magnetic field of external coils with the field produced by a current driven through the plasma itself. The dominant approach to magnetic confinement.
- StellaratorA machine that confines plasma using only the magnetic field of intricately twisted coils. Unlike a tokamak, it does not need a large current flowing through the plasma.
- Inertial confinementCrushing a small pellet of fuel with powerful lasers or similar drivers, so that fusion happens in the tiny interval before the fuel flies apart.