So far in this book on radio-frequency plasmas the properties of plasmas have been investigated in the absence of periodic time-dependent parameters or boundary conditions, therefore effectively in a DC steady state. In this chapter the restriction to DC conditions will be relaxed to prepare the ground for the discussion of plasmas that are sustained by radio-frequency (RF) power supplies. Although quantities such as electric fields and potentials then become a combination of steady and periodic values, there are many useful situations that appear to be (RF) steady states when viewed over many cycles – all relevant quantities exhibit coherent oscillations and identical conditions are reproduced within each cycle. When the plasma is sustained by a combination of volume ionization and surface loss, and the response of ions is restricted by their inertia, as is the case in many RF plasmas, the density structure of the plasma shows barely any temporal modulation. The ion space charge in sheath regions is similarly robust. That is, the density profile of the plasma and that of the ions in the sheath remain steady. However, because the electrons are much more mobile, they are able to respond virtually instantaneously, thereby changing the spatial extent of sheaths and quasi-neutral plasmas. The potential profile is related to the spatial distribution of charges through Gauss's law, and this will change in line with applied potentials and consequent rapid redistribution of electrons.

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.