1 - Electrical Discharges and Plasmas —A Brief Tutorial

Abstract

This chapter explains some of the basic concepts of plasmas and electrical discharges. The presentation is informative and conceptual, concentrating on presenting results and their qualitative physical explanations. In an electrical discharge, current is conducted between two electrodes by electrons in plasma of ionized gas or vapor. The motion of the electrons is limited by collisions with atoms and ions. Some of the collisions may ionize atoms, which is crucial for maintaining the plasma in the face of losses. The collective concentration of charged particles and their motion can cause electrical and magnetic fields, which in turn can further influence the concentration or motion of the plasma particles. One collective effect of great consequence is the tendency of the plasma to be neutral-imbalance in the positive and negative charge densities create an electric field that drive the plasma in the direction of neutrality. In electrical discharges, the interaction of the plasma with the electrodes and walls often plays an important effect. Boundary layers, or sheaths, mediate the current flow at electrodes, and often support electrical fields much higher than exist in the plasma bulk. Neutral metal atoms may be emitted from the electrodes or walls by sputtering or evaporation. The cathode region supplies the rest of the discharge with an electron current approximately equal to the circuit current. This is accomplished in glow discharges by the formation of a large cathode fall potential which multiplies the meager cathodic emission of electrons by electron impact ionization of gas atoms near the cathode.