Confinement

Confinement refers to holding plasma at temperatures exceeding 100 million degrees for long enough to sustain fusion reactions.

Simple Definition (High School)

No container can hold plasma at 100 million degrees. Any material wall would cause the plasma to cool before the wall itself melts, stopping the reaction. This is why we need a way to hold the plasma suspended in space without letting it touch the walls. That is confinement. The Sun accomplishes this with its immense gravity, while devices on Earth use magnetic fields or the plasma’s own inertia.

Types of Confinement

Magnetic Confinement

This method uses the property that the charged particles making up the plasma spiral along magnetic field lines, holding the plasma in a cage of magnetic fields.

• Tokamak - A doughnut-shaped device that uses a plasma current
• Stellarator - A device that creates twisted magnetic fields using external coils alone

Inertial Confinement

Powerful lasers or similar tools rapidly compress and heat a small fuel pellet. The reaction is completed during the very brief moment before the fuel flies apart (the time it holds its shape due to inertia).

Gravitational Confinement

This is the method by which stars confine plasma with their own gravity. It cannot be achieved on Earth.

Energy Confinement Time (Undergraduate and Above)

The quality of confinement is measured by the energy confinement time $\tau_E$. This is a measure of how long the plasma’s thermal energy is retained without heating, and together with the density $n$ and temperature $T$, it appears in the Lawson criterion.

$$n \, T \, \tau_E \gtrsim 3 \times 10^{21} \ \text{keV} \cdot \text{s} / \text{m}^3$$

Learn More

• Confinement Methods (Learning Roadmap)
• Lawson Criterion
• Plasma Transport

Related Terms

• Plasma - What is being confined
• Tokamak - A magnetic confinement device
• Stellarator - An alternative magnetic confinement method
• Fusion Reaction - Why confinement matters