A plain-language guide to the fourth state of matter — the hot, electrically charged gas that makes up 99% of the visible universe and is the medium in which fusion reactions occur.
Heat a solid and it melts into a liquid. Heat the liquid and it boils into a gas. Keep heating the gas and something new happens: the atoms themselves break apart. Electrons are stripped away from their nuclei, creating a soup of free electrons and positively charged ions. This is plasma — the fourth state of matter.[1]
Fusion requires temperatures of 100–200 million degrees. At these temperatures, all matter is plasma. The challenge of fusion energy is confining this plasma — keeping it hot and dense long enough for significant numbers of fusion reactions to occur — without it touching any material wall (which would instantly cool it and be destroyed in the process).
Electrically conducting: Because plasma contains free charged particles, it conducts electricity and responds strongly to magnetic and electric fields. This is why magnetic confinement works.
Collective behaviour: Plasma particles interact through long-range electromagnetic forces, not just short-range collisions. This creates rich and complex behaviour: waves, instabilities, turbulence, and self-organisation.
Quasi-neutral: Despite containing separated charges, plasma is electrically neutral on scales larger than the Debye length (typically fractions of a millimetre in fusion plasmas). Any charge imbalance is rapidly screened out.[2]
A typical fusion plasma (like in ITER) has a temperature of 150 million degrees, a density of about 1020 particles per cubic metre (100,000 times less dense than air), and is confined for several seconds. Despite its extreme temperature, a fusion plasma has the density of a good laboratory vacuum — you could wave your hand through it if it weren’t for the temperature.[3]