Plasma Physics - RF Discharges, Sheaths

[maverik]

Hi all, exams in a few weeks and having a bit of trouble understanding the characteristics of plasmas/sheaths. E.g. the following question from a sample paper

"Describe the characteristics of the sheath that forms at the boundaries of confined plasma. In your answer state what is meant by the bohm velocity, explain what a pre-sheath is and why it is needed and why the sheath is dark. Discuss RF sheaths for the case where the RF frequency is less than the electron plasma frequency and greater than the ion plasma frequency. In your answer state when the electrons are able to flow ad describe the equivalent circuit of the sheath."

I kind of understand the pre-sheath and bohm velocity, and very slightly the "darkness" of the sheath but not sure about the rest.

Any help at all would be greatly appreciated!

 

[chrisk]

The darkness of a sheath is a result of insufficient electron energy to create excitation of gas molecules hence no spontaneous emission of photons. What do you know about electron and ion plasma frequencies so as to provide a starting point?

 

[maverik]

Ah I see, great thanks. Is this because they lose energy in collisions with the wall?

The only thing I really know about the electron and ion frequencies are that they have to be greater than and less than the RF frequency respectively in order for the electrons in the plasma bulk to have enough energy for a current to flow through it. However I'm not sure what this means at the sheath?

 

[chrisk]

Go to the below website for a good set of notes on plasmas. This should help clear up your questions. As for the dark space, the acceleration of the electrons, due to the applied electric potential, within this region does not produce the necessary velocity to create excited gas molecules. The transition from dark to light is the point where the electrons have enough velocity to produce light because the spatial length of acceleration is longer.

http://farside.ph.utexas.edu/teaching/plasma/lectures/node7.html