A What happens in a H2-plasma if I add CH4?

Hello Community,

I am a masters student in germany and I am currently trying to grow graphene via plasma enhanced chemical vapor deposition. Therefore I use a reactor with a pulsed DC plasma source. In terms of plasmas, I am a total newby. During my tests I made some (at least for me) strange observations. I am using a H2 plasma (800°C, 10kHz frequency, 4mbar, 200sccm H2, build up like a plate capacitor) and my reactor gives me a plasma power of round about 130 W (Power is measured with P=U*I, so electrons that get to the earthed electrode are responsible for the current). In order to grow graphene I need some carbon source. I am using CH4 for that. So if I add some CH4 to my H2 plasma (exactly 5 sccm) the plasma power drops down 35%. If I add more CH4 the power keeps droping until 40 sccm CH4 (240 sccm total). If I continue to add CH4 the power increases again! (All the details are in the attached file).
So my humble understanding of impact ionization is very lmited. I would guess that in my H2 plasma there are mostly positived charged hydrogen ions (protons) because I need free elctrons to generate the plasma power and there is nothing else than hydrogen in there. If I add CH4 these hydrogen ions collide with the CH4 molecules and decomposite the CH4. If I say this "reaction" uses electrons from the surrounding in order to generate neutral H2 again I could understand the dropping in power. But then why is the power going up again?
I probably miss something very important here but my knowledge of plasma physics is very limited.
Do any of you have ideas what happens in my plasma? I don't need an exact solution for this problem but maybe a general understanding.

Thank you very much for your support


Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

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