# I What is the approximate impedance of an RF plasma "antenna"?

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1. Oct 20, 2016

### Voltux

I've searched high and low for data regarding this from scientific papers to books and I cannot find anything in regards to the approximate impedance to excite a vacuum vessel to plasma state.

In particular I want to built a RF Plasma cleaning chamber, however, I am not sure how to design the matching network for the field. I have tried using low power ~2W approximately 20-50ohms impedance, however, I was unable to create a plasma.

I have been reading that the required RF power to excite a gas to plasma state decreases quite a large amount as it approaches a vacuum and then shoots up exponentially as the vacuum level approaches a perfect vacuum.

Anyway, what sort of impedance should I design my RF amplifier with in order to excite some plasma? Does anyone happen to have data on this? Typically I would determine this experimentally, but I lack a tunable matching network and since I plan on designing this amplifier from the ground up it would be easier to get an idea of what impedance to expect!

2. Oct 21, 2016

### tech99

If we pump down a glass tube of air and try to pass a discharge through it, I know from experiment that it requires about 5 kV DC. I suggest using a parallel tuned circuit to step up your power to several kV. It seems probable that the plasma is a non linear load anyway, so the impedance will change with power.

3. Oct 21, 2016

### tech99

I also understand from a colleague that to excite a 12 inch fluorescent tube with RF he requires about 10W of RF or more.

4. Oct 21, 2016

### Voltux

From my understanding the excitation of plasma has different mechanisms for DC breakdown vs RF. I believe the RF is a means of magnetic excitation whereas the DC breaks off an electron or something like that. I can't remember off the top of my head.

I have done the DC method and I believe I used about 3kV under vacuum in a glass bottle. I'm sure this differs depending on distance. I've also heard that you can more easily ignite plasma via RF if you have a heating element to send out electrons to "strike" the plasma.

The problem with the DC method was that my connectors got so hot to the point it was melting my vacuum tubing, and cracked my glass vessel. I'd like to try the RF method using a 13.56MHz RF source. The problem is that I've heard everything from 3kOhm to mOhm impedance and I'm not sure how to go about that. I just recently tried to ignite a plasma ball with a few watts but I had nothing.

So I'm looking at designing a small 15W RF Amplifier to play around with for testing, and then building a 100W system once I understand the matching bit more so I don't waste a \$30 FET. I'd love to start some research into thin films, deposition, and play around with plasma cleaning!

5. Oct 21, 2016

### tech99

You could experiment with DC and RF operation of the little NE2 neon bulb, which requires about 100 volts.