Plasma Guns

[Minuteman37]

Ok let's say becouse of Totallynotspacemagic (TM) I can produce self contained balls of superheated plasma and when the surface tension of these plasma balls is broken they expel they're contents.

How hot would that plasma need to be to become a weapon? Assuming the process to make the plasma ball self contained uses minimal energy how expensive/ resource intensive would weapons grade plasma balls be? Considering plasma has minimal mass how hard would it be to prepell these plasma balls through air/a vacuum? Would railgun/Gauss technology be needed?

[starslayer]

Originally Posted by Minuteman37

Ok let's say becouse of Totallynotspacemagic (TM) I can produce self contained balls of superheated plasma and when the surface tension of these plasma balls is broken they expel they're contents.

How hot would that plasma need to be to become a weapon? Assuming the process to make the plasma ball self contained uses minimal energy how expensive/ resource intensive would weapons grade plasma balls be? Considering plasma has minimal mass how hard would it be to prepell these plasma balls through air/a vacuum? Would railgun/Gauss technology be needed?

1. The temperature would have to be well on the other side of 'insane' because the density is so low, much hotter than the surface of a star. Your basically trying to answer 'how hot do I need to get a single puff of air to do appreciable damage (except the puff of air will have radically higher density than a plasma ball); remember plasma is a state of matter more excited than a gas, it is very sprace.

2. Magnetic acceleration is a must because your 'totally not magic' balls break if something disturbs their surface tension, so only magnetic propulsion will work.

[Anthony]

Originally Posted by Minuteman37

Ok let's say becouse of Totallynotspacemagic (TM) I can produce self contained balls of superheated plasma and when the surface tension of these plasma balls is broken they expel they're contents.

How hot would that plasma need to be to become a weapon?

Depends on quantity of plasma. It's most a question of total energy, anything hot enough to form a plasma will be dangerous in sufficient quantities.

Originally Posted by Minuteman37

Assuming the process to make the plasma ball self contained uses minimal energy how expensive/ resource intensive would weapons grade plasma balls be? Considering plasma has minimal mass how hard would it be to prepell these plasma balls through air/a vacuum? Would railgun/Gauss technology be needed?

Depends on the technobabble being used.

[Minuteman37]

Originally Posted by starslayer

1. The temperature would have to be well on the other side of 'insane' because the density is so low, much hotter than the surface of a star. Your basically trying to answer 'how hot do I need to get a single puff of air to do appreciable damage (except the puff of air will have radically higher density than a plasma ball); remember plasma is a state of matter more excited than a gas, it is very sprace.

2. Magnetic acceleration is a must because your 'totally not magic' balls break if something disturbs their surface tension, so only magnetic propulsion will work.

But I can't heat gas to useful temperatures becouse like ice to water it will just transform into a plasma.


Can't plasma be condensed? That in of itself isn't spacemagic (TM)?

[Ulzgoroth]

Originally Posted by Minuteman37

But I can't heat gas to useful temperatures becouse like ice to water it will just transform into a plasma.


Can't plasma be condensed? That in of itself isn't spacemagic (TM)?

Most things can be compressed with enough pressure, plasma definitely included.

Your confinement technobabble just needs to be ludicrously powerful, is all. Par for the course.

[Agemegos]

Originally Posted by Minuteman37

Can't plasma be condensed? That in of itself isn't spacemagic (TM)?

It can, but that takes colossal pressures. The core of the Sun is a plasma up to about 150 times as dense as water, but the pressure that keeps it in that state is about 265 billion atmospheres.

The other thing about dense plasmas is that they are as hot as a Madras curry and as opaque as Postmodernist prose. Therefore they radiate a ferocious amount of thermal radiation — at that sort of temperature we're talking about UV and soft x-rays rather than IR. Which means they cool rapidly, especially if they have a large ratio of surface area to volume, i.e. if they are in small pieces.

Your plasma bubbles have to be contained by something that is ludicrously strong and ridiculously insulating, opaque, and prefectly reflecting. Well into spacemagic.

[Ulzgoroth]

Originally Posted by Agemegos

Your plasma bubbles have to be contained by something that is ludicrously strong and ridiculously insulating, opaque, and prefectly reflecting. Well into spacemagic.

Uh, how fast is fast, on the cooling? Most plasma guns don't need the bubbles to remain viable for more than maybe a small handful of seconds, and often only fractions of a second.

[Agemegos]

Originally Posted by Ulzgoroth

Uh, how fast is fast, on the cooling? Most plasma guns don't need the bubbles to remain viable for more than maybe a small handful of seconds, and often only fractions of a second.

I thought the magazine was full of stored plasma bubbles.

Okay, the photosphere of the Sun is about 6000°K, but it's only 3% ionised. Let's say that a really-o, truly-o hydrogen plasma is one in which the average thermal energy of an ion equals or exceeds the first ionisation energy of the species. For hydrogen that means about 12000°K, or twice as hot as the surface of the Sun. (That's going to be about 50% ionised.)

As you recall, the Stefan-Boltzmann law of radiation says that the power of thermal radiation goes with the fourth power of the temperature. The plasma balls are going to radiate 16 times as bright as the surface of the Sun (and by Wien's displacement law its output is going to peak at 250 nm, which is UV-C). The Sun puts out 63 MW per square metre, so a blob of 50% ionised hydrogen thermal plasma would radiate about a gigawatt per square metre.

How long is a piece of string? Let's consider a plasma ball 0.01 m in radius, on the grounds that anything wider than that can't be fired out of a gun that will fit in a shoulder holster without spoiling the cut of your jacket. Surface area = 1.26 × 10^-3 m², so the thing will be an eye-searingly bright blue-white ball that radiates 1.26 megawatts, about half of it ionising radiation.

Okay, that's a bit ferocious. A plasma ball (of 50% ionised hydrogen thermal plasma) 1 mm in diameter radiates only 3.15 kW.

How fast does its temperature come down, then? That depends on how dense it is. Let's pick a value more than that of the surface of the Sun (2 × 10^-4 kg/m³) but less than that of the core of the Sun (1.5 × 10^5 kg/m³), such as 1.0 × 10^4 kg/m³. Our 20mm bubble of plasma would mass 0.042 kg and our 1mm bubble about 5.24 × 10^-6 kg/m³. Now we need to know the heat capacity of a thermal hydrogen plasma, and here things get a bit hair. Ions are monatomic and the heat capacity of a monatomic gas is 12.5 J/mol/K. But the energy of the system in thermal equilibrium will be equally partitioned among the modes, with is 1 for the ions/atoms and 0.5 for the electrons (because we assumed 50% ionisation) and 1 for the entrapped photons (because the plasma is opaque). Call it very roughly 38 J/mol/K. A mole of hydrogen ions or atoms is 0.001 kg, so those bubbles are 42 moles and 0.00524 moles respectively. The big bubble has a heat capacity of 1596 J/K, so at 1.26 MW it will cool at about 790°K per second (instantaneous). The smaller bubble has a thermal capacity of 0.199 J/K, so at 3.15 kW it will cool at 15 800 K per second (instantaneous).

Note that those figures depend enormously on the assumed size of the bubbles and density of the plasma, also that only I assumed only 50% ionised, which is not hot for a plasma. A highly-ionised plasma would be hotter and radiate much faster.

Also, that's a hydrogen plasma at 12 000 kelvin compressed to the density of lead. The pressure would be about 10^12 pascal, which is about 100 million atmospheres. The rate of cooling will be inversely proportional to the pressure.

[Ulzgoroth]

Originally Posted by Agemegos

I thought the magazine was full of stored plasma bubbles.

I didn't, but you could be right. If so that's certainly going to require what you said to avoid the magazine's contents chilling down and the magazine (and immediate neighborhood) melting a bit.

Originally Posted by Agemegos

The big bubble has a heat capacity of 1596 J/K, so at 1.26 MW it will cool at about 790°K per second (instantaneous). The smaller bubble has a thermal capacity of 0.199 J/K, so at 3.15 kW it will cool at 15 800 K per second (instantaneous).

So for those particular semi-arbitrary numbers the big ball is pretty much fine for the flight time of a small-arms shot (but may be unpleasant to be anywhere near) while the little ball is going to be severely range-limited unless it's traveling extremely fast.

[Agemegos]

Originally Posted by Ulzgoroth

So for those particular semi-arbitrary numbers the big ball is pretty much fine for the flight time of a small-arms shot

Yeah, it's about the size and mass of a 12-gauge shotgun slug, so the recoil ought to be kind of manageable at 500m per second or so. It'll cross the 50m of a typical smallarms encounter in 1/10 of a second, cooling negligibly (80 K) and radiating 126 kJ of ionising radiation (as much energy as about an ounce of TNT, the detonation of an anti-personnel landmine).

(but may be unpleasant to be anywhere near)

Indeed. The big problem is that once containment fails that 42 grams of hydrogen is going to expand from a 1-centimetre radius at 100 million atmospheres to a 4.64-metre radius at 1 atmosphere, very fast. The shockwave will be awesome.

while the little ball is going to be severely range-limited unless it's traveling extremely fast.

Quite.