ABSTRACT
Plasma is often called as the fourth state of matter. Generally, a solid substance melts into a liquid when the temperature is increased at a fi xed pressure. The liquid then transforms into a gas as the temperature is further increased. Under high temperature, the molecules in the gas decompose to form atoms that move freely in random directions in space. If the temperature is further increased, the atoms decompose into freely moving charged particles, and the substance enters the plasma state. Thus, under certain conditions, a plasma can be thought of as a collection of electrons, singleand multiple-charged positive and negative ions along with neutral atoms, excited particles, electromagnetic radiation, molecules, and molecular fragments. In plasma, the densities of the excited particles, ions, and electrons as well as the intensity of the electromagnetic radiation far exceed those that are found in more mundane situations encountered elsewhere. An important feature of plasma is that the positive and negative particles are in a state of charge equilibrium, and the sum of the positive and negative charges in a suffi ciently large volume is equal to zero. In fact, plasma technology is a dry, environmental-friendly, and cost-effi cient process in a myriad of applications.
To produce a plasma, electron separation from atoms or molecules in the gas state, or ionization, is required. Ionization occurs when an atom or a molecule gains enough energy from an outside excitation source or via interactions (collisions) with each other. In fact, to have a stable plasma discharge, the choice of heating mechanisms and geometric confi gurations for sustaining discharge is important. In most practical situations, the plasma is produced by an electrical discharge. Various forms of discharges such as radio frequency (RF), microwave, glow, and arc discharges have been developed. Generally, these discharges can be separated into two categories: gaseous and metallic plasma discharges. The RF, glow, and cathodic arc discharges are discussed in the following sections in detail.
19.1.1.1 Radio Frequency Discharge
