Figure 2.2 Classification of plasmas according to charge density per unit volume and electron  temperature in degrees Kelvin. Plasmas used in materials processing cover a large  portion of the low-energy density-temperature space.

A plasma is a partially or fully ionized gas containing electrons, ions, and neutral atoms and/or molecules. Plasma science is the study of the nonlinear collective interactions of electrically charged particles with each other, with neutral atoms and molecules, and with electric and magnetic fields. Figure 2.2 illustrates the wide range of electron densities and temperatures that are encountered in nature as well as in the laboratory.

Low-energy plasmas occupy a large portion of the density-temperature plane, with electron densities ranging from 10⁵ to 10²⁸ m^-3 and electron temperatures ranging from 100 to 10⁵ Kelvin. Two regimes in the density-temperature plane are characteristic of plasmas used in plasma processing. One of these includes glow discharges, in which the temperatures of electrons and heavy particles are widely disparate. The second includes thermal plasmas, in which electrons and heavy particles are in approximate thermal equilibrium. These plasmas are classical in nature in that the thermal kinetic energy is large in comparison to the average Coulomb interaction energy. Thus, charged particles usually interact weakly with each other, and electron collisions are usually most frequent with neutral atoms and molecules. The degree of ionization may range from a high percentage to less than 1 part per billion. These plasmas are governed by the laws of collective phenomena and atomic physics. Low-temperature plasmas, particularly glow plasmas, are not in thermal equilibrium and exhibit wide differences in electron, neutral, ion, vibrational, and rotational temperatures. Moreover, within each plasma species there are