Fusion requires an extremely high temperature and sufficiently high density maintained for long enough. In stars, temperatures of millions of kelvin give nuclei enough kinetic energy to approach one another, while high core density increases the collision rate and gravity confines the plasma.
Why Both Conditions Are Necessary
Atomic nuclei are positively charged, so they experience electrostatic repulsion. At very high temperatures, matter exists as a plasma of free nuclei and electrons. The nuclei have greater average kinetic energy and collide at higher relative speeds, making it more likely that they approach closely enough for the attractive strong nuclear force to bind them.
High density places more nuclei within a given volume. This increases the frequency of collisions and therefore the probability of successful fusion reactions. Density alone is insufficient because slowly moving nuclei would still be kept apart by electrostatic repulsion.
| Condition | Why fusion needs it |
|---|---|
| Very high temperature | Increases nuclear kinetic energy, allowing nuclei to approach despite electrostatic repulsion |
| High density | Increases the number of collisions per unit volume per unit time |
| Sufficient confinement time | Keeps the hot, dense plasma together long enough for fusion to release useful energy |
There is no single universal threshold because the required conditions depend on the nuclei involved and the confinement method. As a stellar example, the Sun's core is approximately with a density of about . Its enormous gravitational field provides gravitational confinement and maintains the core pressure and density.
A common misconception is that temperature alone causes fusion. High temperature raises collision energy, but sufficient density and confinement are also needed to produce enough successful collisions.
IB Exam Technique
This is SL and HL content in E.5 Fusion and stars. For an Explain question, link each condition to its consequence: high temperature increases kinetic energy, high density increases collision frequency, and stellar gravity confines the plasma. Do not merely state “high temperature and pressure” without explaining the physical mechanism.
