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In the world of Warhammer 40k, come multiple weapons with variable levels of feasibility, most namely the Bolter and the Chainsword, which are the simplest.

On the other hand, there are other weapons that are slightly more complex, such as the Plasma Cannon. While the real world already have plasma weapons, they are mostly not hot and weak. Warhammer 40k’s Plasma Cannon, on the other hand, is much more powerful, and seems to operate using superheated nuclear plasma:

"Plasma Weapons work by using hydrogen fuel suspended in a cryogenic state, in either fuel flasks or backpack containers. As the fuel is fed into the miniature fusion core inside the weapon, the hydrogen energises into plasma which is held in the core of the weapon by powerful electromagnetic containment fields. When fired, the fields dilate open and the plasma is ejected via a linear magnetic accelerator as a bolt of superheated matter akin to a solar flare in appearance and temperature. This bolt will explode on impact and can generate the destructive heat of a small sun."

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I see that the potential issues is the fuel being suspended in a cryogenic state, and a fusion core small enough to fit in the gun, in contrary to the giant tokamaks and spheromaks in modern times, and will be the main parts of its feasibility which I would like to question. The size might also be an issue, and I fear that this cannon will not likely be feasible handheld, and can only be if it is of artillery size.

Other than that, the other components of the Plasma Cannon are fine and obey current plasma railgun technology.

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  • $\begingroup$ You should limit your question to one weapon. And then if you have a useful answer you can post another/followup question for the next weapon. And so forth. (You technically don't need to wait with the followups, but it's likely to get your whole chain of question more overall attention and thus better answers if you at least space them over, say, a week) $\endgroup$ – dot_Sp0T Dec 7 '19 at 8:29
  • $\begingroup$ Okay, I will rewrite this. $\endgroup$ – CYCLOPSCORE Dec 7 '19 at 8:35
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    $\begingroup$ edit the existing question instead of deleting and reposting. $\endgroup$ – L.Dutch - Reinstate Monica Dec 7 '19 at 8:47
  • $\begingroup$ Nah, already done so. Just get to the new one. $\endgroup$ – CYCLOPSCORE Dec 7 '19 at 9:10
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    $\begingroup$ Please follow the rules. Circumventing the reputation system by deleting downvoted post and reposting them as new is NOT appreciated. $\endgroup$ – L.Dutch - Reinstate Monica Dec 7 '19 at 9:30
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Plasma guns, as traditionally envisaged, don't work and can't work.

Hot things expand. Gasses under pressure that are not confined expand. The plasma bolt will expand very rapidly upon leaving the barrel of the gun, and a very hot, high pressure and rapidly expanding cloud of gas is traditionally known as a fireball, or possibly an explosion.

The 80s SDI-era MARAUDER weapon used a railgun to accelerate a plasmoid to about 1% of the speed of light, so that it could reach a distant target before expanding into uselessness. The amount of energy required to do this trick involved building-sized capacitor banks. There's a sharp limit on how far these things that be miniaturised because very strong electromagnetic fields in small spaces will cause arcing and dielectric breakdown which will wreck the gun in short order. The amount of energy that the MARAUDER would deliver to the target wasn't much more than, say, a 30mm cannon shell, but was a hell of a lot more inconvenient. If you're not shooting a target in space, the range is wasted, and if you're shooting a target in an atmosphere the plasmoid will experience air drag and will slow down faster, meaning that it stays pretty useless by the gun has got a lot bigger, heavier and more expensive.

Forget plasma weapons in an atmosphere. It isn't obvious that pulse plasma weapons in space are much use either, compared to neutral partical beams or lasers.


With regards to this specific design, other than the fact that plasma weapons won't work, I'd think twice about carrying a portable "fusion core", whatever one of those might be. Fusion reactions either release a lot of neutrons, or a lot of x-rays, and often both. You can't shield yourself against those without a magical forcefield which 40K has but real life does not.

Hydrogen is very low density, even when frozen. You'd be better off using something that's a lot more dense, and a lot easier to ionise.

A reflex sight should look down the barrel. That's just a regular telescopic sight.

Well done to the designers for using a linear accelerator though. That's probably the only sensible and plausible bit of the whole thing.

Really, if you have the sort of power supplies and technology that might allow you to make this kind of thing, you'd be better off making railguns, coilguns or lasers, or maybe just miniature hypersonic missiles with micro-sized antimatter-catalysed nuclear warheads on them. Similar effects without the need for nearly so much handwavium, and they can be guided too. What's not to like?

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  • $\begingroup$ The minitaure hypersonic missiles is the standard boltgun round, he mentioned. 40k is wild. $\endgroup$ – James McLellan Dec 7 '19 at 11:26
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    $\begingroup$ @JamesMcLellan nah, they're not hypersonic, or nuclear, or guided. They're just gyrojets in an overengineered housing. $\endgroup$ – Starfish Prime Dec 7 '19 at 13:36
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Since there is "far future science" and possible meta-magic, it might be better to work outside-in, than inside-to-out.

What's the Difference in Effectiveness Between Lasers and Plasma?

Lasers send a fire-hose of photons downrange, some of which get sponged up or reflected by powdery particulate mix of whatever atmosphere you are shooting through. Absorption causes blooming (the air turns into plasma) at low energies (about 1MJ per cubic centimeter) which provides a satisfying lightning/gunshot like sound effect, but costs you focus as that fire-hose of zero-mass photons is sent spraying all over the place.

Plasma's particular improvement to this idea is mass. Mass actually has a stupid amount of energy it brings to the target ($E = m c^2$), but plasma doesn't take advantage of all of that, just the extra momentum and deeper cutting.

Plasma is a similar fire-hose, firing through a similar column of powdery environment particulates to hit it's target.

Isaac Newton, in trying to estimate the penetration of ballistics, recognized that at high velocities the dominant term was the acceleration of the material that the projectile needed to move out of the way. Because of this realization, he came up with a handy (and generally right order-of-magnitude) approximation :

$ depthOfImpact = {{densityOfProjectile \times lengthOfProjectile} \over densityOfTargetMaterial} $

40K magic and meta magic aside, we have something we can examine, but we're not ready to yet because this hydrogen reserve could be at an arbitrarily high pressure, which increases it's density $ pressure \times volume = density \times constant \times temperature $ However, we have information that will help us put upper limits on this density.

We know that having the plasma highly pressurized means that hot plasma is going to spray all over the place when it exits the muzzle. 40K seems to acknowledge this, as the weapon has a 16% chance of killing the person firing it when "overloaded".

We know that the plasma gun is being fired by a Space Marine - a genetically modified, cybernetically enhanced, practically immortal, semi-artificial human being. And, because that's just not ridiculous enough, it's a Space Marine wearing a strength and mobility enhancing exoskeleton. And not just any new, mostly human marine (Scout) carries this equipment. Only front line marines that are mostly finished with the enhancement process are issued these weapons.

Why Do We Care? Conservation of Momentum

Despite all of the far future tech and pseudo magic, 40k has never indicated that Space Marine armor has any kind of stability compensation. As a result, we can assume that the weapon can not be spraying forward any more momentum $momentum = mass \times velocity$ than it would take to lay the Marine on his backside for firing it.

This weapon is meant to be fired while running (but not a full sprint), and without an particular bracing or much time to aim. Let's imagine that the power armor will transfer the gun recoil into its exoskelton, so that the entire mass of the Marine (about 435 kilograms in armor, per codex) can be used to absorb the recoil, although it might take no more than one-third of that to knock the Marine off-balance. A Marine is noted as moving as fast in all of this equipment as a typical human infantryman (5 miles per hour = 2.2 meters per second)

Using those limitations, the maximum recoil of the plasma gun is $435 kg \times 2.2 m/s \times 30\% \approx 287 $ kg m/s. That is the maximum possible recoil of our plasma gun, independent of future technology and space magic.

What's That Mean? Figuring Out the Plasma Density

This is an anti-armor weapon, intended to punch through state-of-the-art far future plate.

According to the 40K rulebooks, the plasma gun has a fairly short scale range of 24 inches at a 1 inch = 0.5 meters scale (12 scale meters). It seems engineers are trading range for cutting capability by maximizing plasma density.

Given, again, that this weapon is designed to be fired multiple times by running, assume the plasma velocity is no less than triple the range (${{12 meters} \over {1 \over 3} seconds} = 12 \times 3 = 36 $ meters per second)

From our maximum momentum, then, we can calculate the total mass of a distinct plasma "shot" : ${{287 kg m/s} \over {36 m/s}} \approx 8 kg$

This weapon works primarily by drilling through the target material, like a laser. According to the 40K codex, it can be fired rapid fire (twice per second), and while on the move. That gives us a maximum dwell time on target of $1 \over 3$ seconds, and probably should be much smaller (but we'll use $1 \over 3$ for now).

By using the ejector velocity and the dwell time, we can calculate the projectile pulse length of $36 m/s \times {1 \over 3}seconds = 12 meters $

The muzzle area of the plasma gun looks about equal to a boltgun, which is cited in Lexicanum literature as about 50 caliber (12.7 millimeter diameter). The muzzle is $3 \over 4$ the diameter squared, 0.000120 square meters.

We've got everything we need now to compute the plasma density! $8 kg = density \times 0.00012 \times 12$ or $density = {8 \over { 0.00012 \times 12 }} = {8 \over 0.0014} = 5,714$ kilograms per cubic meter.

How Reasonable Is This?

In real-world fusion experiments, we're attempting to hit target densities of $10^25$ ions of deuterium, each ion weighing $ 2 \times 1.67 \times {10}^{-27} $ kilograms. Combining the ion density and ion mass to get a mass density $ 2 \times 1.67 \times {10}^{-2} = 0.0334$ kilograms per cubic meter.

This means that the 40K fusion gun, as described in 40K lore is producing plasma densities about 200 thousand times higher than in stellar fusion experiments.

Is that reasonable for a "far future" society that has fusion cells in their lunchboxes? Possibly. For 21st Century Earth, however, it's well outside what is currently possible.

What if You Went the Other Way?

What if, energy costs be damned, you tried to plough through with relativistic velocities?

The relativistic momentum equation is $momentum = Lorentz \times {mass}_{initial} \times velocity$

The Lorentz contraction factor increases to near infinity as the velocity approaches the speed of light ($3 \times 10^8$ meters per second), but the velocity contribution remains roughly unchanged.

This is all still capped by the maximum recoil the shooter can take of 287 kg m/s.

Without worrying too much about plasma density and rounding up to 300 kg m/s for recoil, the Lorentz contraction factor $1 \over \sqrt{1 - {v^2 \over c^2}}$ is a relativistic mass of ${3 \times 10^2 kg m/s} \over {3 \times 10^8 m/s}$ = $3 \times 10^{-6}$ kg weight of a plasma "shot"

Skipping the re-evaluation of every step to the final equation, and substituting in the new "shot" weight ${3 \times 10^{-6}} kg = density \times 0.00012 \times 12$ or $density = {{3 \times 10^{-6}} \over { 0.00012 \times 12 }} = {{3 \times 10^{-6}} \over 0.0014} = {4.2 \times 10^{-9}}$ kilograms per cubic meter.

That is way below the 0.0334 kilogram per cubic meter plasma density current technology is capable of! This might be a useful weapon!

With Newton's approximate and the plasma density, though, how far deep can this beam cut through air? The density of air is about 1 kilogram per cubic meter. Newton's approximation, then, would be : ${{{4.2 \times 10^{-9}} \times 12 meters} \over {1}} \approx {4.2 \times 10^{-8}} $ meters.

No. That's not going to work either.

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  • $\begingroup$ "Regular" humans apparently use the guns too (example). Plasma guns are described as firing "bolts", not beams or streams (see the OP), so they don't drill so much as go "bang". You can't really use Newton's impact depth approximation of hypervelocity projectiles, let alone charged particles, though given the shaky foundations here it probably won't make much difference. You might look up the "continuously slowing down approximation" if you were interested, though. $\endgroup$ – Starfish Prime Dec 7 '19 at 14:00
  • $\begingroup$ Thanks for the tip! $\endgroup$ – James McLellan Dec 7 '19 at 15:35
  • $\begingroup$ @James McLellan this is excellent: walking thru the physics and laying out the math. I learned from your answer. More like this please. And a takeaway: even a flaky question about scifi impossibilities offers an opportunity to learn some real things. $\endgroup$ – Willk Dec 7 '19 at 16:02
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    $\begingroup$ Thank you. I'd like to try the Imperial lasgun. I'm curious, now, how it would turn out. $\endgroup$ – James McLellan Dec 7 '19 at 18:24
  • $\begingroup$ @Willk - added a question for the lasrifle $\endgroup$ – James McLellan Dec 7 '19 at 21:52
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Well I will be direct the weapon that describes:

“ Plasma weapons work by using hydrogen fuel suspended in a cryogenic state, either in fuel flasks or in backpack containers. As the fuel is fed to the miniature fusion core inside the weapon, hydrogen is energized in the plasma in the core of the weapon through powerful electromagnetic containment fields. When triggered, the fields open and the plasma is ejected through a linear magnetic accelerator like a ray of superheated matter similar to a solar flare in appearance and temperature. This ray will explode upon impact and can generate the destructive heat of a small sun . ”

This n or would work in reality EVER ! There are at least 100 the problem that already mention (at the least the most important ) but in any way the lists will

1º The plasma bolt will expand very quickly when leaving the gun barrel,

  1. the process created as minimum x - ray if it is melting also open a storm neutron ( radiation ionizing + many neutrons = rich rich cancer of terminals [suponi in not die of radiation poisoning clear do])

3rd heat, only being less than 100 meters from where impact that thing you leave between good gold and piece of coal steaming (definitely you're going to go from good juices but at the least do not see rojito to the cut piece) is saying literally says. This ray will explode upon impact and can generate the destructive heat of a small sun. A small sun !!! You know how much heat is going to release that.

4th the plasma undergoes rose viscous with no air I will calculate mainly because I laziness see the caliber canonical that coda to calculate the area of the plasma bolt and also need to know the temperature to know which is the density of the effective air ( that is , having finds that the air and any gas decreases its density how much may its temperature be )

5th related to the 4th point the plasma will cool for radiation and convection (and really not be pleasant heat if either a space open probably will not rostice or very uncomfortable [would be like being in a day of summer very warm or like being next to a big fire like using a flamethrower ] in a dry space, it would be a matter of taking a few shots to be in a furnace that is not deadly [assuming that whatever you shoot is far enough so that the explosion does not kill you, it is one of the previous points] if it would be really uncomfortable, the real problem here is precisely that the plasma is going to heat the air and most likely this makes it spread at supersonic speed you know what it is called that commonly. explosion or in this case a boom supersonic as to make the aircraft to the break the barrier of the sound)

the thermodynamics more basic says it's a bad idea to be close to something very hot because of the heat is processed to the environment fortunately our plasma shot is not big enough (regarding your area ) to rostizarte nothing more than shoot it

I could continue with minor problems such as the energy that the weapon would require and the risk of firing it in battle but the question was not all the problems with the plasma gun . Era Viability of Warhammer 40k plasma cannon

And that I will answer is Viable ? how is it raised? not for a billion different reasons

Is there a way to be viable ? yes but oh what to make changes to its operation that I will explain

Is it practical ? N or , as is practiced as effective ar ducks with one Barrett M82 50 (a rifle anti materials)

notice that from this point I am not responsible for their bullshit this is a THEORETICAL way in which this weapon would be possible with our current technology and if any light tries to build this weapon and is killed it is not my legal responsibility nor will I be responsible if it works and kills allege said this let's start

Let's start by taking the parts of the explanation that make sense

With what we have left something like this

Plasma weapons work by using hydrogen fuel suspended in a cryogenic state, either in fuel flasks or in backpack containers. As the fuel is fed to the miniature fusion core inside the weapon, hydrogen is energized in the plasma in the core of the weapon through powerful electromagnetic containment fields. When triggered, the fields open and the plasma is ejected through a linear magnetic accelerator superheated matter similar to a solar flare in appearance is fired

That is the part that makes sense but still there are some details to correct

Hydrogen if in some way very likely deuterium (because the tritium is unstable) the one that specifies the cryogenic storage is good in that way it is actually stored in the rocket fuel tanks and thus small gas cartridge is also stored as The co2 now to solve problems like the one that the gas would dissipate the solution is shoot vortices, vortices how does that help us ? A vortex is in broad strokes a fluid that revolves around an axis can be a whirlpool or a toroid depending on the orientation of its axis the property that interests us is that a vortex can travel great distances practically without dissipating for this case I will use toroids because they are easier to do (but it is possible to make the plasma swirl) the weapon that could work would be something like that

The plasma is generated in a railgun (two parallel conductive rails that do not touch in which a high electrical voltage is created) that is fed by the discharge of a bank of capacitors of high capacitance (there are small capacitors and values of several farads so I assume that this part is the easiest to do) it is important that they are not super capacitors that although with better capacities its discharge is very slow, deuterium gas is injected (let's leave it in a gram so that the toroid has sufficient density to strike strong but does not require large [ridiculously large] amounts of energy less would barely have an effect and more simply requires ridiculous energy for a gun) the deuterium will be injected at a great speed and accelerate further into the railgun then move on to a chamber (extremely ceramic) heat resistant) with a superconducting coil on the outside (the coil will be submerged in liquid helium and made of cuprates helium goes through a Stirling piston cooling system using the reverse Stirling cycle and dissipating the heat created in the process with sodium and beryllium bronze heatsinks) the result of each shot will be a plasma toroid at 10,000 degrees Celsius (the deuterium is fused in a self-sustaining way [when the energy that is created is equal to or greater than that spent by the fusion] is 40,000 degrees Celsius) the radius of the barrel will be 20 mm the same as in a current 20 mm caliber weapon and the toroid will go about 20 meters per second similar to a paintball gun at that speed the shot will reach a fairly decent distance to be a gun if you want a way to load more the shot would be an inertial confinement fuser in front of the railgun To add super hot plasma but it is an unnecessary part and that gives more problems than advantages the battery for the weapon would be an aluminum ion battery since it is capable of storing much energy ogy and is extremely resistant to damage even if you want more energy can be used Battery s nuclear is or RTG ( Radioisotope Thermoelectric Generator ) (not to be confused RGT nuclear batteries because they are different) although this I leave to choice attached a diagram though very basic by the way I am a Spanish speaker and I had to use the google translator so the translation will not be very accurate enter image description here

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