A plasma may be produced in the laboratory by heating a gas to an extremely high temperature, which causes such vigorous collisions between its atoms and molecules that electrons are ripped free, yielding the requisite electrons and ions. A similar process occurs inside stars. In space the dominant plasma formation process is photoionization, wherein photons from sunlight or starlight are absorbed by an existing gas, causing electrons to be emitted. Since the Sun and stars shine continuously, virtually all the matter becomes ionized in such cases, and the plasma is said to be fully ionized. This need not be the case, however, for a plasma may be only partially ionized. A completely ionized hydrogen plasma, consisting solely of electrons and protons (hydrogen nuclei), is the most elementary plasma.
Plasma is created when the electrons of the atoms or molecules in a neutral gas are liberated.
There are three primary methods:
- Direct current (DC) gas discharge – a voltage is applied between a cathode and anode and free electrons are accelerated. The energized electrons collide with and ionize neutral atoms.
- Alternating current (AC) or Radiofrequency (RF) discharges – a time-varying voltage is applied to a set of coils which creates a time-varying magnetic field. From Faraday’s law, a time-varying electric field is generated which accelerates electrons that collisionally ionize the gas.
- Photoionization – sometimes a laser of the right frequency can liberate the electrons from atoms. The frequency depends on the elements in the gas.
There are other ways of liberating electrons, of course, but these are the most commonly used methods in terrestrial plasma applications.