In the previous chapter fundamental equations were established that govern the properties of low-pressure plasmas. Elementary processes such as collisions and reactions were described, and fundamental electrodynamic quantities such as the plasma conductivity and the plasma permittivity were derived. These concepts were mostly considered in the context of an infinite plasma or else were viewed as part of a global system without reference to the internal structure of the plasma volume.

Laboratory plasmas are confined. The consequence of the presence of boundaries on the structure of an electrical discharge through an electropositive gas will be discussed in this chapter. The basic idea to keep in mind in the discussion is that in this case charged particles are predominantly produced in the plasma volume and lost at the reactor walls. This was the basis of the global balances in the previous chapter. Conditions in the central volume may differ to some extent from those near the edge. Close to the walls a boundary layer spontaneously forms to match the ionized gaseous plasma to the solid walls; whether insulators or conductors, the walls have a major influence.

Figure 3.1 is a picture of a discharge generated between two parallel electrodes by a 13.56 MHz power supply. The discharge appears to be stratified, with regions of different properties. Light is emitted from the central region, with evidence of internal structure particularly away from the main vertical axis.