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I'm a university student so I have a relatively limited knowledge of such physics concepts, however I have researched fusion reactors and ion engines, both theories I incorporated into my design.

Context

I am thinking about this as I am writing a sci-fi novel. In terms of the technology of my fictional society, we're talking around another 40 years into the future + another world war (so more rapid advances in military technology). I'd envision the plasma gun as a small and portable anti-armour weapon.

the design as follows:

Xenon gas is held within a canister in the grip of the gun. It is sprayed into a torus-shaped chamber, where it is ionised by an electron gun and rotated around it at high speeds by electromagnets and heated rapidly. This also has the effect of compressing the plasma, and the exact amount of compression would be decided by a computer. Once it has reached the ideal conditions, firing of the rifle would be allowed, and on pulling the trigger, the toroidal chamber would open and the plasma would be ejected across a railgun, which would further accelerate it to speeds beyond the speed of sound and out the barrel.

what I want to know is, would this work and therefore: what would be the projected range, how small could it be reduced to, what would be the required power? And if it doesn't work, is there any other physics concepts that could be applied that would fulfil these requirements - specifically as a plasma gun?

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  • $\begingroup$ Welcome to Worldbuilding! On this site it is expected for the querent to perform reasonable research before asking a question. In particular, what is unclear on the Energy Weapons Sidearms at Atomic Rockets? (You do know about Atomic Rockets, right?) Have you looked through the many, many questions about plasma guns on this site? P.S. What are "these" requirements? Did you forget to add them? $\endgroup$
    – AlexP
    Apr 4, 2021 at 8:07
  • $\begingroup$ requirements is misleading, I'll try and fix that $\endgroup$ Apr 4, 2021 at 8:25
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    $\begingroup$ We ask that questioners wait at least 48 hours before awarding the "best answer" tick, as otherwise our international membership may be discouraged from providing additional valuable answers. $\endgroup$ Apr 4, 2021 at 10:30

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TL;DR: plasma weapons just aren't really very good. Yours seems unlikely to compare favourably against almost any other kind of weapon.


Here's the problem with guns that fire plasmoids like this: hot, pressurised gas expands.

In order for your plasma gun to be useful, the plasmoids it throws are gonna have to be quite warm... tens of thousands of kelvin, maybe. They're also going to have to be quite dense, because a blob of low density gas is going to have pitifully low momentum and so is going to be unable to bash its way through the dense atmosphere you might find at eg. sea level on an Earthlike planet.

A hot, dense blob of gas might also be called a "fireball" or even "explosion". In a vacuum it will expand at something like the Alfvén velocity or ion acoustic velocity, which are likely to be measured in hundreds of kilometres per second. This process will happen more slowly in an atmosphere, but not that much more slowly because your plasmoid needs to be dense in order to push through the air and so the relative density of the atmosphere will seem pretty low.

Clearly you need to project your plasmoid at velocities exceeding its expansion rate, and that's gonna mean at least 100 km/s, and that means you need a pretty fearsome accelerator in order to accomplish the task in a compact weapon.

How much power will this all need? Well, its very hard to say, but the only real world example of a plasmoid weapon that we have is MARAUDER, which used a power source called SHIVA STAR. This used a multi-megajoule capacitor (not too inconvenient) running a multi-hundred-kilovolt levels (that's more inconvenient) and capable of discharging in about a microsecond, yielding peak powers on the order of terawatts. That's going to be exceptionally difficult to miniaturise without creating something that will immediately blow itself to bits, likely killing the operator and/or vehicle it is mounted on.

MARAUDER threw its plasmoids at several thousand kilometres per second, and they hit with the force of a few kilograms of TNT. It won't have any substantial ability to penetrate armour, because the bang isn't very big and the density of the projectile will be far lower than conventional armour-penetrating rounds and shaped-charge jets. MARAUDER was intended for use against Soviet ICBMs which are a bit more delicate than even lightly armoured vehicles or buildings.

Lets compare that briefly with something like an antitank rocket launcher, such as the classic bazooka. These can throw a couple of kilos of warhead a few hundred metres fairly promptly. You can build it with 1940s technology, it can survive use and abuse by GIs, it has been proven to be cheap and effective even against heavily armoured targets. Modern descendants like the Javelin are guided and can wreck even the latest generations of heavy armoured vehicle.

It would make more sense to use guns, railguns, lasers, rocket launchers or electron beams instead of shooting plasmoids... basically anything else is likely to be better.

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  • $\begingroup$ thank you for the response, You have destroyed my plasma gun hopes and dreams with cold facts and logic. Thanks to your input I have discovered I poorly researched the actual effects of plasma guns, and have concluded like you that they are effectively useless in the scale I want them, and shall move on with the novel to more realistic concepts such as railguns. sorry for wasting your time and brainpower on such a lost cause. $\endgroup$ Apr 4, 2021 at 8:34
  • $\begingroup$ Did Marauder ever manage to propel its plasma bolts at a distance of more than a few meters? $\endgroup$
    – AlexP
    Apr 4, 2021 at 8:36
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    $\begingroup$ @AlexP not that I'm aware of. It is possible that it has accomplished more interesting things since it all went thoroughly classified in the mid-nineties. A cynical observer might imagine that its claimed performance was as credible as any other bit of the SDI program, which is to say "not at all". $\endgroup$ Apr 4, 2021 at 8:53
  • $\begingroup$ This is a good answer, but I disagree with: "In a vacuum it will expand at something like the Alfvén velocity or ion acoustic velocity". This is true in many cases, but there are configurations you can make of plasmas that are in MHD equilibrium and in a vacuum/uniform heat bath would theoretically last forever without dissipating (ignoring kinetic effects). In reality, if they're shot through the air they'll dissipate due to instabilities driven by interactions with air molecules and radiative cooling-- the confinement time is controlled by the time scales associated with these processes. $\endgroup$ Apr 4, 2021 at 13:38
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    $\begingroup$ Additionally, the plasma doesn't necessarily need to be super dense to move through the atmosphere-- another option is that if you crank up the particle velocity super high, the stopping power goes down and they just won't exchange much momentum with the air. The downsides to this are that one, the same is true with momentum exchanged with your target, and two, that a quick glance and NIST's pstar database suggests you would need energies on the order of 100's of MeV to get a 1 km range, which means your weapon is more a particle accelerator than a plasma gun. $\endgroup$ Apr 4, 2021 at 13:44

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