The uses of plasma
Okay, what’s plasma? It’s an ionised form of matter similar to a gas, or a gas which has lots of ionised particles in it (to put it a bit simply). Most people will have heard of plasma-screen TVs, or maybe plasma being used in experimental fusion reactors such as ITER. But there’s more than that.
Artificial plasmas include your TV (if you’ve got a plasma-screen, which I don’t) or fluorescent lighting in more everyday life, and this may include plasma brushes in dentistry. More exotic or specialist varieties include plasma torches, arcs from Tesla coils, or even in ITER, like I mentioned earlier. Of course, the problem with fusion is that we’ll need to wait a while before it becomes practical: research has been going on for over 50 years and it’s still quite hard to get it to work. The general idea with fusion reactors is that lighter elements (anything with an atomic number less than iron (56), which appears to be the cut-off between fusion and fission) are fused together at high temperatures, releasing nuclear binding energy. The inherent advantage in this is that if the temperature is not maintained, the plasma cannot sustain itself and the reaction will be quenched by itself – this means it’s much less likely to go catastrophically wrong.
But plasmas can occur quite naturally. Lightning is one type which can reach a temperature of around 30000°C – for comparison, the surface of the Sun is around 6000°C! Ball lightning is another phenomenon, characterised by spherical lights in the sky accompanied by a thunderstorm, and it has been suggested that this might be a form of plasma – if it even exists, that is.
But if you really want beautiful naturally-occuring plasmas, then you need look no further than the polar lights. What happens here is that charged particles from the solar wind in the higher levels of the atmosphere collide with each other; the collisions produce photons with a particular frequency/wavelength in the visible spectrum. Photographic film and possibly digital cameras are sensitive to a higher range of frequencies than the human eye, so they can see more colours. Why am I mentioning this? Well, photos often show more red in aurorae than the humans themselves saw, mainly because the red photons produced are just outside the range of the human eye.
Plasma often shows up in science-fiction as weapons, usually as a form of superheated gas or liquid that causes some nasty burns – almost like a futuristic form of napalm. Some works use this as hand weapons (Covenant weapons in the Halo series, for instance), or as ship-to-ship weapons. I think the latter is slightly more likely, but I wouldn’t exactly hold my breath about it: plasmas need to be confined to a particular space to prevent dissipation, and even if that weren’t a problem, there’s other obstacles. For instance, such weapons would generate a LOT of heat (dual plasma rifles overheating in Halo 2/3, anyone?), and even today we have materials that can withstand such levels of heat. They were used in the Space Shuttle, and the ceramic plates used by some bullet-resistant vests. Finally, blackbody radiation means that they would lose their heat very rapidly and, because they’re glowing, there is a risk of blinding the operator.