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Plasma

Plasma is the “fourth state of matter,” following solid, liquid, and gas. It refers to an ionized gas in which electrons have been stripped from atoms, creating a mixture of freely moving electrons and ions.

Simple Definition (High School)

Section titled “Simple Definition (High School)”

When you keep heating a gas, collisions between atoms become more and more violent, and eventually electrons are knocked out of the atoms. The resulting “soup of electrically charged particles” is plasma. The inside of a fluorescent light, lightning, auroras, and the Sun itself are all made of plasma. Most of the visible matter in the universe is in the plasma state.

Precise Definition (Undergraduate and Above)

Section titled “Precise Definition (Undergraduate and Above)”

In physics, an ionized gas is called a plasma when it satisfies the following three conditions.

  1. The size of the system is much larger than the Debye length (charge imbalances are screened, so the whole is quasi-neutral).
  2. A large number of particles exist within a Debye sphere (collective behavior dominates over individual collisions).
  3. The plasma oscillation frequency is higher than the collision frequency with neutral particles (the electromagnetic response dominates).

Because of this, plasma is electrically conductive and responds collectively to electric and magnetic fields. The plasma in a fusion device exceeds 100 million degrees and is fully ionized.

Role in Fusion Research

Section titled “Role in Fusion Research”

For atomic nuclei to overcome their electrostatic repulsion (the Coulomb barrier) and fuse, the fuel must be heated to over 100 million degrees. At this temperature, matter inevitably becomes plasma, so fusion research is itself the study of how to create, heat, and confine plasma.

Learn More

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