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