Classification of Plasmas

Based on the relative temperatures of the electrons, ions and neutrals, plasmas are classified as “thermal” or “non-thermal”. Thermal plasmas have electrons and the heavy particles at the same temperature, i.e., they are in thermal equilibrium with each other. Nonthermal plasmas on the other hand have the ions and neutrals at a much lower temperature (normally room temperature), whereas electrons are much “hotter”. A plasma is sometimes referred to as being “hot” if it is nearly fully ionized, or “cold” if only a small fraction (for example 1%) of the gas molecules are ionized, but other definitions of the terms “hot plasma” and “cold plasma” are common. Even in a “cold” plasma, the electron temperature is still typically several thousand degrees Celsius. This Handbook will help in understanding of plasmas has led to important advances in fields as diverse as computers, lighting, waste handling, space physics, switches and relays, and lasers.