What is plasma?

Heat a solid hard enough and it melts into a liquid. Heat the liquid and it boils into a gas. Keep heating that gas — past tens of thousands of degrees — and something stranger happens: electrons rip free of their atoms, leaving a soup of positively charged ions and free electrons. That soup is plasma, the fourth state of matter, and it is by far the most common state in the universe.

More than 99% of visible matter — every star, every nebula, the solar wind, the aurora — is plasma. On Earth it's the medium inside lightning bolts, neon signs, fluorescent tubes, plasma TVs, and welding arcs. It's also the medium that every fusion company on the planet is trying to confine long enough, hot enough, and dense enough to extract net energy.

Why plasma matters for fusion

Fusion happens when two light atomic nuclei (typically deuterium and tritium — two heavy isotopes of hydrogen) get close enough that the short-range strong nuclear force overcomes their mutual electrostatic repulsion and they fuse into a heavier nucleus. The reaction releases enormous energy — about four million times more per gram than burning coal.

The catch: nuclei only get that close when they're moving fast enough, which means temperatures above 100 million Kelvin — roughly six times hotter than the core of the Sun. At those temperatures, matter is unavoidably ionised. Plasma is not a design choice in fusion. It is the only state of matter in which fusion can occur at human-relevant rates.

The Lawson criterion

For a reactor to produce more energy than it consumes, the plasma has to satisfy a simple-looking inequality known as the triple product: n · τ · T — the plasma density (n), the confinement time (τ), and the temperature (T) all have to be high enough simultaneously. Push any one too low and the others can't make up for it. Most of fusion's engineering history is about climbing the triple-product mountain.

Three ways to hold a star in a bottle

Plasma in everyday life

Most plasma you encounter is "cold" — only the electron component is heated, while ions and neutral atoms stay near room temperature. That's how a fluorescent tube can glow with a 10,000-K electron temperature without melting itself. Industrial plasmas etch silicon wafers, sterilise medical equipment, and cut steel. Fusion plasmas are at the opposite extreme: thermal equilibrium across ions and electrons at hundreds of millions of degrees.

Where to go next

• The Science journal — three decades of fusion breakthroughs
• Glossary — every plasma physics term, defined
• Company directory — who is building what, and how