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Plasma weapons are a staple in many science fiction settings. Guns hurl brightly colored balls of plasma towards enemies. These balls then explode, injuring or killing the target. Superficially, this seems reasonable. Plasma can have many different colors, it certainly produces light and it is rich in energy, so it could potentially be useful as a weapon. Furthermore, in laboratories, people have produced long, thin, slender beams of plasma. What, apart from practical matters such as the size and energy requirements, obviously created by a wonderful science fiction source of plentiful power, from using plasma as a weapon?

In order to answer this question, let us first consider what a plasma is. A plasma is a gas in which a significant fraction of the molecules is split into ions and electrons. These electrons are hot and are responsible for the light show a plasma can give. The principle of our plasma weapon would be to create a ball of this plasma stuff and hurl it at our enemies.

Let's start with the easiest case: a plasma gun in a vacuum. In a vacuum, the plasma doesn't interact with anything. So, we can release a puff of gas, and ionize it using perhaps a powerful current. If we give it a push out of the barrel, it will start flying towards our enemy. Simple, no? Not really. There are two major problems with this.

First of all, our plasma will radiate. This gives a nice light show, but unfortunately for us, cools the plasma down rapidly. Assuming we create a 10 cm plasma ball, with a temperature of 10,000 K, we can compute the heat loss using the Stefan-Boltzmann law. This heat loss is about 20 MW. The question is how long we can sustain this heat loss before our plasma cools down. If the plasma is fully ionized, and has a density of 1023 particles per cubic meter - this is comparable to the density of air, we have around 3x1021 particles in our plasma. Let's assume our carrier gas is argon - other gases give a very similar answer. The ionization energy of argon is about 16 eV1 per atom. Doing a bit of math, we find that our plasma has a whopping 5 kJ of energy. In other words, it will burn out in less than a millisecond.

Worse, it's not much of a weapon. 5 kJ is roughly enough to make a very tiny cup of espresso. Not completely harmless; if it were to strike you, you'd get a nasty burn, but not nearly as dangerous as, say, a .50 BMG.

I've conveniently forgotten that our plasma will expand rather rapidly. Fortunately, the time scale at which this happens is substantially longer than the sub-millisecond it takes for the plasma to cool down due to radiation. As such, provided we can launch our plasma to really high speeds, this shouldn't be a problem, at least, not as much of a problem as the radiation.

Having seen that the only way to feasibly create a plasma gun is by making the plasma go really fast, we probably need to worry about the effectiveness of such a weapon in the atmosphere. In principle, our plasma is a puff of gas that is not even very dense. If we shoot it in air, the air molecules will act as a brake on the plasma. Worse, the plasma and the air will mingle, cooling down the plasma even more rapidly. As such, a plasma gun doesn't seem feasible in atmospheric conditions.

Is there nothing our superior sci-fi technology can do about this? Well, that is going to be difficult. Once the plasma has left the gun, our super-duper sci-fi technology probably can't touch it. The only thing we could try is putting a magnetic bottle around the plasma - never mind how you would project this. This could stop the plasma from expanding. Unfortunately, it doesn't solve the cooling problem, nor does it solve the fact that the atmosphere is in the way. All in all, it doesn't seem likely we can build a nice plasma gun.

The fact that we probably can't make a cool sci-fi plasma gun doesn't mean that we cannot use plasmas to cause havoc. In particular, we could try to use a laser to ionize air, hence creating a plasma. Plasmas conduct electricity. As such, we could try to electrocute our opponents. A bit crude, and it wouldn't work against enemies that don't conduct electricity. Still, it's better than a plasma gun that has less range than a pointy stick.

In conclusion, the annoying tendency of plasma to spread its energy all over the place makes it difficult to turn this into a practical weapon. Only under vacuum conditions and using a ultra-high velocity gun could we hope to do any meaningful damage. A more practical application would be to use the plasma to conduct a lethal dose of electricity to our target.

Sources:

1. http://www.chemicool.com/elements/argon.html

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