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

 

[paranormal]

Sure, I can provide some insight on this topic. Plasma physics is a complex and fascinating field, so it's understandable that you may be struggling with some of the concepts. Let's break down the different aspects of the question and discuss them one by one.

First, the sheath is a thin layer of plasma that forms at the boundaries of a confined plasma. It is essentially a transition region between the plasma and the surrounding material, such as a wall or electrode. The characteristics of the sheath can vary depending on the properties of the plasma and the surrounding material.

The bohm velocity refers to the average velocity of the ions in the sheath region. It is named after physicist David Bohm, who first described the concept. The bohm velocity is important because it determines the thickness of the sheath and the amount of energy that ions can exchange with the surrounding material.

The pre-sheath is a region of plasma that forms before the sheath. It is necessary because it acts as a buffer between the plasma and the sheath, allowing the plasma to adjust to the electric field and preventing it from directly interacting with the sheath. Without a pre-sheath, the plasma would quickly lose energy and collapse.

The darkness of the sheath refers to the fact that it does not emit much visible light. This is because the sheath is a low-density region and does not contain enough particles for efficient light emission. It is also influenced by the electric field in the sheath, which can prevent particles from recombining and emitting light.

Moving on to RF discharges and sheaths, the behavior of the sheath can change depending on the frequency of the applied RF (radio frequency) signal. If the frequency is lower than the electron plasma frequency, the electrons are unable to respond to the signal and the sheath remains relatively unchanged. However, if the frequency is higher than the ion plasma frequency, the ions are able to respond and the sheath can become more dynamic.

In terms of the equivalent circuit of the sheath, it can be thought of as a capacitor in series with a resistor. The capacitor represents the electric field in the sheath, while the resistor represents the flow of particles through the sheath. The point at which the electrons are able to flow is known as the transition frequency, and it can be calculated based on the properties of the plasma and the surrounding material.

I hope this explanation has helped to clarify some of the concepts related to plasma physics