But what, exactly, are plasma weapons? For those who are not already aware, plasma is usually described as the fourth state of matter, after solid, liquid, and gas. More technically, it is ionized gas, ie- gas in which the energy level is so high that the electrons will not stay confined in their atomic energy shells so they escape. The Earth's Sun is largely composed of plasma, which can also be described as a hot "soup" of free-floating nuclei and electrons. Therefore, a plasma weapon would logically be something that fires plasma at the target.

However, particle beams fire ions at the target, yet they are generally called particle beams, not "plasma weapons". So what is the distinction? It seems to be that a plasma weapon is primarily a heat-based weapon, ie- it is the internal energy of a hot plasmoid which damages the target, not the forward kinetic energy of the ion stream.

Indeed, so-called "plasma weapons" in sci-fi generally fire visible "bolts" which move far, far slower than the particles of a hot plasma would move. For example, a typical hand-held "plasma weapon" in sci-fi will fire a bolt that moves at 1 km/s at the most, or may even be subsonic, yet even a relatively "cold" 1 eV plasma will have an average (root mean squared) particle velocity of 13.8 km/s for nuclei and 593 km/s for electrons (assuming even energy distribution). This is a major impediment to their effectiveness and an incomprehensible "feature"; why would one even want a plasma weapon where the particle velocities are all randomized in a slow-moving confined blob, rather than being directed forward at great velocity as they would be in a particle beam? Such a weapon would be far less penetrative by its nature, hence far less efficient even if it works.

And these weapons generally have one other fascinating on-screen trait: they do not appear to be affected by gravity. This is not a small quibble; dense objects like bullets drop in gravity, and light objects like helium balloon float up due to buoyancy. You can't normally see a bullet dropping because it is too small and fast to see with the naked eye in flight, but the arcing is appreciable and significant, yet it is not present in sci-fi "plasma weapon" blasts, which fly straight and true to their targets as if there is no gravity at all. One could attempt to rationalize this with a projectile that has the density of air, but if it has the density of air, then it would have the aerodynamic properties of a cool air balloon, which would make a poor projectile to say the least.

How well would a Plasma Weapon work?

The short answer is: at any range where it takes more than a thousandth of a second for the bolt to reach the target, not too well. You see, plasma spreads very quickly, and while plasma guns actually do exist in real-life1, and have been proposed as a mechanism for replenishing the fuel burn-up fraction in a Tokomak-style fusion reactor for steady-state operation, they have never been seriously considered as a weapon. They can fire a "blob" of plasma in the MJ-range, but this blob would not stay together for much distance in vacuum, never mind atmosphere where they would run into a virtual brick wall (sea-level atmosphere is billions of times denser than a fusion plasma). You could extend the range by hurling these ions out of the barrel at an extreme velocity (eg- relativistic), but those moving bolts we see in sci-fi do not appear to be traveling anywhere near that quickly.

All right, so why don't we just confine the plasma? Well of course, there's the obvious objection that a blob of plasma will not confine itself, so you'd have to create some kind of magical containment field which moves with the bolt and requires no technological apparatus to sustain itself. But it gets worse. Let's say we're talking about a 1 metre long bolt with a diameter of ½ cm, and a yield of 1 MJ (equivalent to roughly 4 ounces of TNT). Let's say it's 1 keV plasma (roughly 8 million K); you would need 6.24E21 ions, ie- less than 0.01 grams of hydrogen plasma. Small problem: air would be many times denser then this plasma, so the bolt would tend to fly up because of buoyancy, and it would need some kind of propulsion system to drive it through air because it certainly isn't going to coast through atmospheric resistance on its miniscule momentum. Both of these problems can be alleviated through sheer particle velocity (a sufficiently hypersonic projectile will have enough momentum to mitigate buoyancy effects and extend its range). But since that would be more of a particle beam than a sci-fi moving-blob "plasma weapon", it is not applicable here. In short, a typical subsonic or marginally supersonic sci-fi moving-blob plasma blast would require a magical self-contained containment field, and it would float up into the air even if it did hold together.

In short, ask yourself how well a "hot steam gun" would work. Doesn't sound all that impressive, does it? You visualize a cloud of steam shooting out of a gun and promptly dissipating in the air. So why does it sound like such a great idea when you replace "steam" with "plasma", which is just a really hot gas?

Can you make a Plasma Weapon work?

OK, why don't we try solving this problem by using a much lower-energy plasma with increased density? We could try to solve the buoyancy problem by making it colder (say, 1 eV, or 8000K, which is a bit hotter than the surface of the Sun), thus necessitating a thousand times more ions in the same volume, but its density would still be much too low to push it through the atmosphere on momentum alone. It wouldn't necessarily float up, but try throwing a balloon at someone and you can see how well an object with atmospheric density would fly if hurled at the target.

No, if you want it to push its way through atmosphere on momentum, it must be either much denser than air or moving at extreme velocity, which sci-fi plasma weapons generally do not (and which would make it more of a particle beam than a traditional sci-fi "plasma weapon"). So what if we decrease the volume to make it as dense as a solid projectile? Well, that takes care of the "can't push its way through atmosphere" problem, but now you have to make it tiny, and in order to do that, you need to squeeze it with immense pressure. If we squeeze our 1MJ plasmoid into a 1cc volume and apply the ideal gas law (which is a good model for plasmas), we find that the pressure is in the range of 700 GPa! When you consider the fact that this is a thousand times greater than the yield strength of high-grade steel, you can begin to see the problem.

How many problems arise when you need a containment field a thousand times stronger than steel just to hold your plasmoid together? Some questions leap to mind, such as "if they can create such a strong containment field which somehow supports itself and doesn't even need a projector device, then why can't they make personal shields as strong or even stronger?" One would also have to ask why it doesn't glow like the Sun, since it would be hotter than the Sun's photosphere and denser than steel. And finally, one would have to ask what the point is of this whole speculation, since our plasma "bullet" is now denser than aluminum and should act like a real bullet now, which means it should drop in gravity. While that may not be an insurmountable hurdle for a hypothetical sci-fi weapon, it certainly doesn't match the sci-fi weapons we know, which do not arc noticeably in gravity.

In conclusion, the idea of a slowly moving self-contained plasmoid weapon simply doesn't make any sense. Your "bolt" is constantly trying to blow itself apart on the way to the target, you must invent some kind of ridiculously strong yet easy-to-run containment field to make it hold together (thus raising obvious questions about why this super containment technology is not used to effortlessly protect against these bolts), and when it finally does hit the target and the mythical "containment field" shatters, the barely-contained ions within will promptly scatter in all directions, thus wasting the majority of their energy by dissipating harmlessly into space. Even those ions that do strike the surface of the target will achieve poor penetration; most of their kinetic energy is randomized rather than being directed forward, and the gas cloud lacks the characteristics which would allow it to push through solid armour rather than simply heating its surface. And after all that, the plasmoid won't move in a straight line the way they're invariably shown in sci-fi; it should arc downward in gravity, just like the 30mm auto-cannon shots.