Reactor Influence on Plasma Behavior

In astrophysical studies, one can study plasmas unaffected by solid surfaces. By way of contrast, laboratory plasmas always interact with such surfaces. Accordingly, if we are to properly understand the behavior of laboratory plasmas, we must inquire into the nature of the plasma-solid surface interaction. 

There are several aspects of this interaction that we will touch on. First, we will review concepts of the DC and AC discharge. Then, the consequences of using unequal size electrodes will be discussed. For AC discharges, frequency will also play a role and finally, the influence of magnetic fields on discharges will be considered. 

There is also a potential difference between the positive column and tube wall. This potential difference is created because the electrons are much more mobile than heavy ions and tend to flow rapidly out toward any bounding surface. Since the tube wall is an insulator, they tend to collect there causing the insulator to assume a negative potential relative to the plasma. This creates an electric field close to the tube wall which hinders further electron flow towards it. A deficit of electrons forms in a sheath close to the surface, and this sheath assumes a net positive charge. Ions in the plasma, however, see the tube wall potential which is negative compared to the plasma and are attracted to it. This is the diffusion to the tube walls mentioned in the previous paragraph, and is often referred to as ambipolar diffusion. 

Finally, when an AC discharge is operated with a blocking capacitor between the power supply and one of the electrodes, that electrode will assume negative self bias. Such an average negative voltage on this electrode can serve to accelerate ions toward it with considerable energies. 

It is interesting to note that this is the ion bombardment that is used in plasma ( dry ) etching to promote the anisotropic character of that process, it is also important in the understanding of sputtering phenomena.13 

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