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Plasma weapons are a common trope in sci-fi, but they all suffer the major problem of "cool but inefficient", the plasma dissipates too quickly to be used as an effective weapon.

Based on this answer, I devised a concept of plasma containment, but I'm not sure if it's effective enough

enter image description here Behold my paint skills

The concept is that the hydrogen plasma is coated with a:

  • magnetic layer, that's used for containment, could be accelerated by a railgun.
  • And a thin dielectric mirror layer, that reflects the thermal radiation of the hydrogen plasma.

Would this containment be able to keep the plasma alive for long enough to be effective? (600m range, projectile speed is 2 km/s, projectile volume is a 2,5 cm diameter sphere, density and temperature is an exact match to a stronger (computer controlled) plasma cutter's exhaust)

Mephistopheles
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    If you're firing a projectile at 2 km/s, you don't need the plasma -- just the kinetic impact will do plenty of damage by itself. That's about Mach 6, depending on altitude and temperature. – Salda007 May 28 '17 at 11:50
  • @Salda007 It depends on the target. – Mephistopheles May 28 '17 at 11:51
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    Let me put it this way: If you hit your target with a projectile moving at 2km/s in an atmosphere, you'll be hitting them with plasma -- the projectile will be moving fast enough to ionize the air as it passes through it. So the plasma bow shock will hit the target moments before the projectile does. – Salda007 May 28 '17 at 14:18
  • @Salda007 That's true, but what really matters is the specific heat of the material, the plasma is made out of. – Mephistopheles May 28 '17 at 18:58
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    Why define it to this level? Science Fiction is also filled with fiction. We all bow to Arthur C Clark's third law re advanced tech same as magic. Could you explain how a jet engine works to Vikings? Or Neanderthals? Don't worry, you won't have to. Ever. Same here: soldier, take gun, point, shoot, reload or not. – dcy665 May 28 '17 at 21:12
  • @Salda007 Not arguing the sense of launching plasma at 2km/s, but accelerating matter at 2km/s would require a tremendous amount of energy. At least this plasma is pure energy. Arguably getting pure energy to move 2km/s is move at that speed is far easier than matter. – Neil May 29 '17 at 09:48
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    @dcy665 We sometimes want to define things to a far greater level of detail than actually gets used in a work, simply in order to better understand the limitations of the thing ourselves. By understanding the details ourselves, the story being told can be more consistent. – user May 29 '17 at 11:02
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    @Neil "At least this plasma is pure energy." Eh, what? OP specified hydrogen plasma. Hydrogen is mass. Granted, it doesn't have a great deal of mass, and it's gaseous at most temperatures that might be encountered (and in the case of the OP's scenario, a plasma -- obviously), but it's still mass. – user May 29 '17 at 11:04
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    Wait... what? The XCOM series already has a hard sci-fi explanation of how the plasma rifles work. Why would you then show up, overrule that, and say "Nope, that does not work! I have a better thing!"... only to then suddenly go onto WB SE to ask if it works. I mean... why would you assume that 1) the in-game explanation cannot work and 2) your concept can work? What is the reason for wanting up try to upstage and outplay the XCOM authors? – MichaelK May 29 '17 at 13:08
  • It is just intuition, but it seems to me that since a hot plasma always wants to expand due to the motion of it's particles, the maximum of it's ''blowing off power'' is it containment device. (If it has more energy than that (enough to cause serious harm) it will break out before time) But there is a solution: If your people has the (almost magical) power to confine hot plasma in a reasonably sized round under the tremendous acceleration associated with firing a gun, it is trivial to them to swap it for antimatter. THIS will cause a big plasma ball upon impact. – b.Lorenz May 29 '17 at 19:15
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    Xcom has access to a fictional Elerium (element 115) to explain the use of plasma weapons. Can our answers use fictional elements with fictional properties too? – Twelfth May 29 '17 at 20:12
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    @Twelfth If you can find the energy and the half-life of it's nuclear isomer, then yes. – Mephistopheles May 29 '17 at 20:47

2 Answers2

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The main flaw I can see in your containment system is that you're saying the plasma "doesn't touch the coating" and yet also accelerating it to $2$kms$^{-1}$ by the time it leaves the barrel of your gun. I can't imagine any magnetic confinement capable of holding something in place which is being accelerated with such force.

The plasma will hit the back of the container before it can leave the barrel.

Assuming we can accelerate them both at exactly the same rate within the barrel you'll still come up against the problem of any resistance slowing the bullet resulting in the plasma moving towards the front end of the container.

As long as your bullet survives it's journey it will hit with a ridiculous amount of kinetic energy and I don't predict your target will be feeling too well after this.

user38754
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A bullet with plasma inside is not possible.

Magnetic field

Today, the most developed project about keeping plasma in limited space is ITER. This is a really big construction. As far as I know, it's very difficult to keep plasma in some seconds: you need strong magnetic fields which generated by current of kiloampers which requied very thick superconductors.

So in the real world it's impossible to generate such strong magnetic field in bullet size.

Temperature

Think of plasma as extremely hot (thousands and millions degrees) gaze. You can't cover the plasma by any hull in small size - it would be melted.

Ammo storage

As described above you can't hold armed ammoes for hours (in you weapon or extra clip) -> you need to create plasma just before shot. It's hard to imagine how (and from where) you could receive energy to heat up somewhat to plasma in fraction of a second. But this is a point of explanation (or just handwaving) you could do.

Ways of creating plasma

To create plasma you have to spend some energy. If we fall back to common weapons (firearms are some kind of explosion) then we could say the energy of shot is held in powder and this is a chemically stored energy. For nuclear bomb, nuclear energy stored in uranium. I would say in all most cases energy stored in matter by some means. So if we had created way to store energy in piece of matter and then release it in form of plasma then we could put it in a bullet and then shot. But you could want to release this energy close to the target and therefore you shooting not by plasma balls, you create them at a target instead.

Shell weight and shell complexity

Let's look at Iowa ammo H.C. Mk 13. Initial speed is 820 m/s which more or less comparable. Weight of explosion is only 8% of full weight of shell and 92% is metal hull. It's because of extremal overload while shell escapes the gun. Note that I use data for explosive shell which doesn't require to penetrate anything before boom.

Also in your picture there are no any magnets, cooling system and/or Dewar vessel. It'll also require additional space and weight so for plasma itself there are almost nothing space.

And yeah - just 0.1 gramm of magnets with speed 2 km/s are already deadly weapon even without plasma

Consider creating plasma close to the target by some means. Heating, explosion, light, alien tech, magic - whatever.

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