Plasma Guns

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Our most iconic science fiction works feature improbably attractive heroes and heroines wielding blaster guns that shoot out energized glowing bolts that zip along at speeds that can be visibly tracked by eye and explode when they hit something. These bolts are often popularly supposed to be made of a state of matter called plasma. Sometimes this is even supported in the show's lore and on-screen terminology. But how realistic are these? Can plasma weapons really even exist?

What is plasma

Plasma is a state of matter where the atoms are not bound to each other and can move freely, and the electrons are not bound to the atoms. Compare this to a gas, which is a state of matter where the atoms are not bound to each other but where the atoms are also electrically neutral. So, to a first approximation, a plasma is simply a gas with some additional electric and magnetic properties by way of having free charges that can transmit electric currents.

You can also compare this to a metal, which is a state of matter where the atoms are bound to each other, either as a solid (like copper) or a liquid (like mercury), but the electrons are free to move. So a plasma will behave something like a metal and something like a gas. But the particular emergent properties you get from both being able to flow and to conduct electricity give it a nature all of its own.

The physics of plasmas can get quite involved. However, for the purpose of this article, we can ask ourselves how much a plasma can deviate from gas-like behavior? This is constrained by the virial theorem, which shows that any localized configuration of fields, charges, and currents cannot hold itself together by any self-forces. It will dynamically expand until it is constrained by external forces.

Terminology: A localized "blob" of plasma is called a plasmoid

Another bit of physics that will be important is the relation between pressure and energy.

• Electromagnetic fields: The pressure of any electric. magnetic, or electromagnetic (like light or radio waves) fields and the charges and currents that produce them is always equal to the energy density (energy divided by the volume) of the fields, currents, and charges.
• Relativistic gas: A gas of particles that is so hot that the particles are relativistic will have a pressure equal to its energy density.
• Ideal gas: A gas of non-interacting atoms, molecules, or other particles that are not relativistic is called an ideal gas. For a gas consisting merely of individual atoms and electrons (instead of molecules or other compound particles) that do not recombine, the pressure is 2/3 of the energy density. If you allow molecules, the rotation and vibration of the molecules can hold additional energy that is not reflected in the pressure – famously, for diatomic molecules like nitrogen and oxygen the pressure is 2/5 of the energy density. But plasmas are usually so hot that molecules cannot form; the atoms are banging into each other so hard that they knock electrons off, and electrons are responsible for chemical bonding, so no molecules are possible. That said, there are sparsely ionized gases (like flame) that have plasma-like properties.
• Internal energy:

Plasma bolts

So the traditional plasma "bolt" as shot from a sci-fi blaster is a plasmoid. Once it leaves the gun, this plasmoid is not confined by any external force except for the surrounding atmospheric pressure. If the blaster bolt is fired in space in a duel between spaceships, it will not be confined at all. Consequently, the bolt will begun to expand in volume as soon as it leaves the gun until it comes to the same internal pressure (due to the kinematic pressure of its atoms and electrons as well as the self-forces due to the fields, charges, and currents inside of it) as the outside pressure.

This, of course, rules out any stable plasma bolt in space-to-space battles. We will get to unstable plasma bolts later, that are not actually held together but just get to their target so quickly that they don't have time to blow themselves apart.

But what about plasma gunfights in an atmosphere. Can you get a plasmoid at ambient atmospheric pressure that can be shot out and which will blow up spectacularly?

Stuff to do

Non-virialized plasmas – neutral particle beams

Producing your plasma at the target (lasers, bombs, hypervelocity kinetics, nuclear explosives)

Visual similarity (tracers and gyrojets)

What about ball lightning