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Low-temperature plasmas are a ubiquitous product of human ingenuity: they light our lives in TL tubes and street lights, they are used extensively to create things from microelectronics to specialty steels to huge turbines, and they are even used for the conservation of archaeological artifacts. The name seems to be a complete oxymoron: how can a weird goo of electrons, ions, molecules, photons, and other exotic unstable species possibly have a low temperature? I mean, you have to feed in tons of energy in your average plasma just to keep it going! We will see where this term comes from when we explore what a low-temperature plasma is.

On Earth, the most common way of making a plasma is by putting a large voltage across a gas, not necessarily at atmospheric pressure. We'll restrict ourselves to noble gases now, because this removes a lot of thorny issues. However, the basic idea is the same for (almost) all gases.

This electric current starts accelerating free electrons, which then collide with the ambient molecules. If they have gained enough energy in the electric field, their collisions can actually knock off the electrons in the molecules, creating an extra free electron and an ion. If this happens enough, a so-called Townsend avalanche ensues. If you can now get your cathode to emit sufficient electrons, you now have a discharge.

This gas discharge will consist of a few electrons and ions floating around in a sea of neutrals, occasionally ionizing one, or getting lost through recombination with an ion, or perhaps just drifting and diffusing to a wall of the plasma, and recombining there.

Now, an odd phenomenon will happen: there are a few electrons floating around in the plasma. These electrons are being tugged on by the electric field, and gain pretty impressive energies, which typically range in the range of several tens of thousands of Kelvins. A useful guide is that one electronvolt (eV), or the energy an electron gains over a voltage difference of one volt, corresponds to 11600 K. Whoa. They lose this energy again in the ionization process, which, depending on the plasma gas, can cost between 2 and 20 eV, roughly. Plasma physicists use electronvolts as a measure of energy. Our idea of slang. Sorry.

Oddly enough, they don't heat up the background gas significantly. This is because of the difference in mass between electrons and molecules, which is at the very least a factor of ten thousand, but for heavy molecules possibly a lot more. Now, electrons transfer a bit of energy proportional to this mass ratio to the molecule per collision, in other words, very little. Hence, the gas stays cold, that is, about room temperature. And that is where the name "Low-temperature plasma" starts to make sense.

You can test this little theory by a simple experiment. Take a small marble. Take a big marble, you know, the ones that are as big as a golf ball. Hit the big marble with the small one. The big one will move just a tiny bit, and the small one will come back. If you do this trick with two identical marbles, you can actually transmit all energy if you hit them dead on-a trick well known to pool players across the globe.

If you increase the power a bit more, the amount of electrons will increase roughly proportional to the power. You see, the loss mechanisms, recombination, drift and diffusion are proportional to the amount of electrons, so twice as many electrons means twice as much power needed to replenish them. Simple, really. The temperature of the electrons tends to drop a bit when you do this, which is not something that can be explained with simple theory.

Along this process, your amount of electrons and ions is getting larger. So large, in fact, that even the inefficient collisions between electrons and ions transfer enough heat to heat your gas to a temperature equal to your electron temperature. At this point, your electron temperature will roughly be equal to one fifth to one fifteenth of the ionization energy of the background gas. My favorite rule of thumb, that.

At this time, another important process starts popping up its head: three-particle recombination. For reasons that are beyond the scope of this write up, electrons and ions don't recombine easily without a third electron being a spectator. This makes this process becoming faster with the third power of the electron density. This puts a ceiling on the electron density for a given pressure and temperature.

However, at this stage, we are getting at a point where the plasma get so energetic it will melt a typical containing vessel. There is not a lot of interest in the region between plasma physics that operate at a range of a few tens of thousands of K, which is basically your garden-variety plasma, like a street light, processing plasma, fluorescent tube, etcetera, and the really hot, tens of millions of Kelvins plasmas.

So, to conclude, "low-temperature" simply means: anything but the million-Kelvin hell of a tokamak. So, one hundred thousand Kelvins still is low temperature for a plasma physicist.

It's perhaps worth noting that there is quite a lot of friction between the high-temperature plasma physicists and the low-temperature folks. Something about tokamaks that don't work and toy plasmas.

And funding, of course.

Sources: I'm a theoretical plasma physicist. Node what you know.

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