What is Plasma?

In essence a plasma is an ionized gas. However, as this book is intended to demonstrate, the behavior of ionized gases is sufficiently different from their non-ionized cousins that it is meaningful to talk of plasma as a fourth state of matter. (The other three states being solid, liquid, and gas.) As is well known, a liquid is produced when a crystalline solid is heated sufficiently that the thermal motions of its constituent atoms disrupt its interatomic bonds. Likewise, a neutral gas is produced when a liquid is heated sufficiently that atoms evaporate from its surface at a faster rate than they recondense. Finally, a plasma is produced when a neutral gas is heated until interatomic collisions become sufficiently violent that they detach electrons from colliding atoms. Heating a plasma does not, however, produce a fifth state of matter. Plasmas resulting from ionization of neutral gases consist of myriads of positive and negative charge carriers whose relative numbers are in the inverse proportion to the magnitude of their individual charges. In this situation, the oppositely charged fluids, which are strongly coupled electrostatically, tend to electrically neutralize one another on macroscopic lengthscales. Such plasmas are termed quasi-neutral (``quasi'' because the small deviations from exact neutrality can have important consequences--see Section 4.16). Strongly non-neutral plasmas, which may even contain charge carriers of one sign only, occur primarily in laboratory experiments, and are not discussed in this book. (Interested readers are referred to Davidson 2001.) In earlier epochs of the universe, all (baryonic) matter was in the plasma state (Longair 2008). In the present epoch, most (baryonic) matter remains in this state. For instance, stars, nebulae, and even interstellar space, are filled with plasma. The solar system is also permeated with plasma in the form of the solar wind, and the Earth is completely surrounded by plasma trapped within its magnetic field (Kallenrode 2010). Terrestrial plasmas occur in lightning, fluorescent lamps, a variety of laboratory experiments, and a growing array of industrial processes. Indeed, the glow discharge has recently become the mainstay of the micro-circuit fabrication industry (Lieberman and Lichtenberg 2005).