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What is Plasma in Physics?

Plasma is one of the four fundamental states of matter and was first described by chemist Irving Langmuir in the 1920s. It consists of a gas of ions atoms that have some of their orbital electrons removed and free electrons. It can be artificially generated by heating a neutral gas or subjecting it to a strong electromagnetic field to the point where an ionized gaseous substance becomes increasingly electrically conductive.

It may be the most abundant form of ordinary matter in the universe, although this hypothesis is currently tentative based on the existence and unknown properties of dark matter. It is mostly associated with stars, extending to the rarefied intracluster medium and possibly the intergalactic regions.

Definition of Plasma

It is an electrically neutral medium of unbound positive and negative particles. Although these particles are unbound, they are not “free” in the sense of not experiencing forces. Moving charged particles generate an electric current within a magnetic field and any movement of a charge that particle affects and is affected by the fields created by the other charges. In turn, this governs collective behavior with many degrees of variation. Three factors define that:

  • The plasma Approximation: It applies when the plasma parameter, Λ, representing the number of charge carriers within a sphere surrounding a given charged particle, is sufficiently high as to shield the electrostatic influence of the particle outside of the sphere.
  • Bulk Interactions: The Debye screening length is short compared to the physical size of the plasma. This criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasineutral.
  • Plasma Frequency: It is large compared to the electron-neutral collision frequency. When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics.