Chapter 6 - THE FLUID EQUATIONS

Abstract

The large majority of the basic processes occurring within gas discharges can be explained on the basis of the fluid equations. The use of the fluid equations suggests that the plasma is considered to act as a whole or as a conducting fluid rather than as individual particles. The condition for validity of this approximation is that the distance between particles be small with respect to the interparticle forces and the mean free path << scale of change of macroscopic quantities. This condition is usually well satisfied in most gas discharges and the fluid equations are found to be useful. There are three main fluid equations: (1) the equations of continuity, (2) momentum conservation, and (3) energy conservation. In a gas discharge, the electron gas and the neutral gas are the main focus. For the neutral gas, the main case of interest is the determination of the spatial temperature distribution when the source of energy is from collisions with the electron gas. Moreover, the Langevin approach to the determination of important discharge parameters gives good results for the electron gas. For the neutral gas, the heat flow terms represent the total heat loss.