I’ve developed a concept for a plasma cannon, and I need it reality-checked.

The cannon would gather ammunition by pumping air into the gun through an external fan. The air would gather inside a chamber, where it would be electrified and converted into high-energy plasma. The chamber would then open into the barrel, and the pressure produced by the plasma would be enough to propel it through the barrel.

Would this concept work? Also, how would the plasma behave once it left the barrel? Would it be concentrated into a single centralized “bullet”, or would it form more of a stream? How far would it be able to go?

Yes

There are several devices in existence that work based on similar principles as yours (with one or two adjustments). While they're used for plasma research, rather than weapons, they could presumably be adapted for your purposes. If you were to make a plasma cannon from scratch, you should probably follow model it at least in part after one of them.

The plasma gun down the hall from me.

My physics department has the SSX lab, a group that studies how plasma structures called spheromaks form and evolve. Here's an image of the main part of their apparatus, the "gadget":

^{Image credit: Michael Brown/Swarthmore College.}

The SSX group is interested in studying how magnetic reconnection in plasma works, and how particular structures (called Taylor states) form and evolve. They, too, have a plasma cannon of sorts, called a magnetized coaxial plasma gun, which works as follows:

1. Hydrogen gas is "puffed" into a chamber (shown above in orange), which has to be extremely clean and must be at very low pressures - about one billionth the pressure at sea level on Earth.


2. A pair of powerful capacitors (one of which is in the image above, in green) are brought to voltages of 5-10 kilovolts each; this is what's needed to ionize the gas.


3. An electric current forms in the chamber and in the plasma itself; by interacting with magnetic fields, a Lorentz force arises and accelerates the plasma down the chamber of the gun. This is the major difference between their design and yours; pressure alone cannot bring plasma to these speeds.

After this, other parts of the assembly create the precise plasma structures the SSX lab is trying to form, but those are irrelevant for your purposes. The important things we can learn from the above mechanisms are:

• You can indeed turn normal (hydrogen) gas into plasma if you have strong enough capacitors, as you're trying to do.


• Magnetic fields and electric currents are needed to accelerate the plasma to high speeds (about 40 km/s).


• Instabilities can arise from impurities in the cannon or the chamber, as can leaks that let air in; because of this, the device needs to be regularly cleaned (with helium, I believe). This, I think, is the biggest issue with a portable plasma cannon; if you want to have an opening for the plasma to exit, you have to worry about stuff from outside coming inside.


• Magnetic fields are also needed to confine the plasma; in their absence (i.e. in your case, after the plasma exits the gun), the plasma would expand and become diffuse, making it a less effective projectile.

The whole assembly also fits in a normal-sized room (well, a pretty crowded and messy room), and should be transportable by a reasonably large vehicle.

MARAUDER

The main issue that would arise from just converting the SSX gadget to a weapon lies in confining the plasma after it leaves the barrel of your cannon. Inside, the hot plasma is confined by magnetic fields in a strong vacuum; outside, you have no magnetic fields (aside from ones arising from the plasma itself) and plenty of sources of turbulence in the atmosphere. This presents a problem, because it seems like it would be very easy for bursts of plasma to diffuse and lose compactness.

This problem might have been solved, though, back in the 1990s, with the start of MARAUDER, a US Air Force project. Its device worked similarly to the SSX lab's machine, with two key differences:

• Plasma acceleration happened via a slightly different design (a conducting probe inside the plasma, it seems).


• The device was explicitly meant to be adaptable as a weapon, rather than a pure research experiment.

After MARAUDER was classified in 1993, however, information about its development became unavailable. It was successful up to that point, and I assume they solved the problem of keeping the plasma projectile together (perhaps simply by shooting it at such high speeds - 30000 to 10000 km/s!).

Given that, I think your plasma cannon could work.

. . . Just please don't take ours. We still need it.

Short answer: no, your design would not work.

TL;DR: don't bother trying to make plasma weapons, especially not for use in an atmosphere.

Longer answer: you can reasonably consider your plasma to be a dense cloud of very hot gas. Your gun doesn't use electromagnetism to propel the plasma, and most people don't hang around in places with particularly intense electrical or magnetic fields, so this is a reasonable simplification.

What happens to a hot, dense gas in air? Well, it will expand rapidly and rise. The hotter and denser it is, the more rapidly and violently it will expand. This probably reminds you of an explosion. It should! What you're suggesting is effectively an explosion in a can that you're holding. The fact that your explosion is made of plasma doesn't mean it will be particularly special in terms of the fireball magically staying the same size and being projected forwards, it'll just go bang.

If you use some other means to propel your plasma out of the gun, it'll travel a short distance but it'll expand and cool so quickly it just won't have any useful range.

An analogy I've seen elsewhere is to compare a prospective plasma gun to a gun that fires steam. Sure, steam can be super hot and super high pressure, but even with the best engineering in the world, that jet isn't going to go very far in air.

Have a look at this, for example. Looks awesome, but its only practical use as a weapon is that the hot gas can be used to propel a projectile. Your plasma gun might look this awesome (and possibly a nicer blue colour) but won't be any more devastating.

http://www.youtube.com/watch?v=_TNSUIsjdpY&t=114

If you want your plasma to go any distance, you'll probably have to look into forming plasmoids (which is basically what the device that HDE showed does). These aren't just blobs of hot ionised gas, but more interestingly structured things that can hold their shape to a certain extent thanks to their electrical and magnetic fields. Not for long mind you... they'll still expand very quickly, but at least you can shoot them out of a gun at a decent speed, so there's a small chance you can hit something before the plasmoid has expanded into nothingness. The US made an experimental device of this sort, but it was massive, surprisingly non-powerful and was intended to be used in space (away from all that pesky atmosphere). You can find more information here: MARAUDER.

Plasma in a vacuum will expand at hundreds of kilometres per second, so it had to project the plasmoid at a good 1% of the speed of light to get any sort of useful range out of it, and that's without any air resistance holding the plasma back.

Here's the capacitor bank that powered the it. You'll be wanting to work on miniaturising that, I suspect.

The guys above have already said it, so FRAME CHALLENGE!

I had this idea in the back of my head for a while but was too busy trying to make dragons overpowered to actually say it.

Patrick Star has already said that plasma is hot, ionized gas, which would rapidly expand in the atmosphere, i.e: an explosion. It's quite devastating, and could potentially damage electronic devices. So, the stuff isn't completely harmless.

A device, that ionizes the gas in the server-side, is doomed to fail as the plasma would break up thanks to instabilities, a mother-booping facility-sized reactor can't completely nullify, and explode into your face like a wet fart.

So, what if you create the explosion in the face of your target? Why not? A canister of hydrogen and som3 graphene supercapacitors, discharging their energy into it through an electro-laser, might let you strike goons down with the Hand of God.

I see some problems in your concept:

1. fans do not build pressure. They just produce a flow of air. If you want to pressurize air, you need pumps.


2. The higher the pressure, the more difficult it is to produce a plasma: the gas molecule will travel a shorter distance before losing energy in an impact, therefore you will need higher energy input.


3. plasma won't move on its own. I have worked for quite some time with sputtering systems, the plasma cloud was not going anywhere in the sputtering chamber. If you want to move the plasma you need something which acts in the same way on the positive and on the negative charges.

It sounds like you have invented a plasma vortex cannon.

The image is from the Backyard Scientist on Youtube. Here is a news clip about a plasma vortex cannon that hopes to be profitable.

“Firefighters won’t go into a building unless they can see their way,”
Faulkner said. “So if they could fire a vortex ring of ionized air
into a space—down a hallway or up some stairs—and clear smoke rapidly,
it would really help...”

The gun forms vortex rings by forcing air or some other gas at high
velocity down the gun’s cylinder. The ring forms when the friction of
the cylinder wall causes a thin layer of the gas to roll forward on
itself like a donut. Imagine a tornado formed into a donut shape. The
ring revolves on itself while traveling out the cylinder and it can
maintain that stability for long distances. Depending on the size of
the gun, Battelle data confirms that a ring vortex can exit a
generator at 90 miles per hour and maintain a speed of at least 60 mph
for more than 50 yards.

https://www.battelle.org/newsroom/press-releases/press-releases-detail/battelle-develops-vortex-ring-gun-for-firefighters-pesticide-delivery

So:
1: Low pressure gases -check
2: Fired under low pressure - check
3: Ionized gas / plasma - sure. Flame is a goo stand-in for plasma.

The difference is that the vortex will confine your plasma into a shape that can traverse some distance and not expend itself in a hot cloud. If your tech can spin the vortex faster somehow it will probably stay together over a greater distance.