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What is plasma?
Plasma is the fourth state of matter — a hot, ionized gas of free electrons and nuclei. It's what fusion fuel becomes at millions of degrees, and what stars are made of.
Understanding plasma is the starting point for understanding fusion: the fuel is a charged, magnetically controllable medium, not an ordinary gas. That property is what makes a tokamak possible. See how hot is a fusion plasma?
Questions & answers
What is plasma?
Plasma is the fourth state of matter, beyond solid, liquid, and gas. If you heat a gas hot enough, its atoms break apart into free electrons and positively charged nuclei — an ionized 'soup' that conducts electricity and responds to magnetic fields. Stars, lightning, and neon signs are all plasma.
Why does fusion need plasma?
Fusion requires nuclei to collide fast enough to merge, which means temperatures of hundreds of millions of degrees — far too hot for atoms to hold their electrons. At those temperatures the fuel is necessarily a plasma.
Why does being a plasma matter for a reactor?
Because plasma is charged, it can be held and shaped by magnetic fields — which is the entire basis of magnetic confinement. A neutral gas couldn't be bottled this way.