What's electropositive ion flux in a plasma?

[Indula]

Is it the same as the plasma density ?
Is it directly proportional to the plasma density ?
Does this term only applies to electropositive plasma or applies to any plasma?

 

[jambaugh]

A plasma consists of positively charged (electro-positive) ions and electrons in most typical scenarios. If you set up an electric or changing magnetic field to induce a current in the plasma then you will see opposite direction force on the electrons and positive ions. You thus get an electron flux in one direction and an electro-positive ion flux in the opposite direction. Typically, since the mass ratio of electron to ion is so tiny the majority of the charge current is the electron motion but the ions move too.

When you ask about proportionality you're going to have to be more specific as to the setting but typically, "yes".

You could imagine cases where the term would not apply because none of the positive charger carriers in the plasma are ions. Examples would be an electron-positron plasma, or a quark plasma, or a the like.

You could also imagine situations where there are also negative ions mixed in with the rest and have an electro-negative ion flux. I have no clue as to whether such could be actualized practically as my experimental knowledge is very limited.

 

[Indula]

Hi jambaugh ,
Thank you very much for your answer.
My experiment tries to build a sensor that can measure the plasma density inside a wafer processing chamber - Plasma Enhanced Chemical Wafer Processing(PECVD) chamber. The process use a cold plasma to deposit or etch the materials on a wafer to create the intricate electronic structures. My goal is to build a simple sensor that can measure the plasma density and help control the chamber parameters. To test the sensor I have built a markup plasma chamber since I don't have any industrial chamber that I can access in my country, Sri Lanka. (Only the final testing can be carried out in an industrial chamber with the support of my supervisors).

Following image shows the plasma chamber I created for the testing. Here I'm using an Argon plasma excited with 75kHz high voltage power.

At the moment my idea is place sensors fixed on a wafer like disc and measure the current generated on each sensor. The wires you see in the above picture will be connected to the sensors. The sensors are mounted as follows, (I'm using an Acrylic disk with several circular copper plates embedded on it. Just for testing and I'm ignoring the contamination effects created due to out gassing that might occur when the ions collide on the plastic plate),

I know that the measured current on each sensor consists of the effect of electrons, ions and displacement current due to plasma sheath. Therefore I might have to go through plasma physics to convert the measured current into plasma density. Since I've only an electronic major this seems arduous and I'm thinking if I can simply take the measured current to be proportional to the ion density on each sensor. I don't know if this is a correct simplification since the voltage used to create the plasma is sinusoidal (For my testing chamber the frequency is 75kHz and for the industrial chamber it's 13.56 MHz). Could you see if the way I'm simplifying my problem is correct or wrong and also any advice regarding my approach to this problem with your knowledge, please ? It would be very helpful for me.

 

[jambaugh]

The problem is that ion density will be proportional to the current and not directly to the density since the voltage potential driving the current. Plasmas behave in a somewhat complex manner, especially rarefied ones so I'm not sure what the relationship between density and resistivity will be. I took a graduate course on fusion physics as an introduction to plasmas but it has a.) been some years and b.) very much specific to the application, namely magnetic confinement in Tokomak reactor parameters. So I don't know the answer to your question.

I found this paper which mentions dispersion interferometry as their method of measuring plasma densities. If you are specifically concerned with ion density in a cold plasma then you may be able to simply measure the radiance of a measured volume of plasma at some optimal frequency. But in general I recommend you just dig through the literature. But maybe someone else around here has better knowledge.

 

[Indula]

jambaugh Thank you for your reply!