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Okay. I have a confession to make. I haven't made plasma - intentionally - since I made a gigantic capacitor back in 2003, when I was in college. Sure there are man-made plasmas all around us: fluorescent bulbs, neon signs, etc. And sure, I've studied what happens to particles in plasmas. And sure, I've accidentally sparked electronic equipment, but I haven't actually played with one in a long while. Perhaps this is why I get an eye-roll when I tell some of my more experienced colleagues that I want to make a plasma.

So I was perhaps unreasonably excited to play around with one of the experiments we have here a DC glow discharge tube. The basic idea is this: a potential is drawn across two parallel plates which are in an enclosed glass tube at low pressure (~ one hundred milliTorr or less). Eventually the gas in the tube - in this case the nitrogen found in the air - breaks down once the voltage across the two plates is high enough and the pressure is low enough. That breakdown voltage is dependent on a few things: the pressure and type of gas in the tube and also the distance between the anode and the cathode.

Upon breakdown, the atoms in the gas get torn apart, so that electrons are freed from the nuclei to which they're bound, forming a plasma. You can actually see this happening as the energy released gets converted into a pink light. Striations in the intensity of the plasma also emerge as the particles crash into each other, gaining and losing energy.

What's really interesting about this system is that when before the gas breaks down, the space between the two parallel plates acts as an insulator, that is to say that it stops the flow of electrons. However, the plasma generates a channel through with electrons can flow, so the capacitor initially set up by the potential difference across the two parallel plates turns into conduit for electricity!

U.S. Department of Energy
Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University.

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