I think that something akin to the Striking scorpion mandiblaster might be a much better way of achieving an accurate, low dispersal plasma weapon.
In a way it's the inverse of using a laser to guide a plasma.

The Mandiblaster is supposed to work by shooting micro needles toward the victim that are heated to form metallic plasma by lasers just before they hit.

By initially firing a pretty standard 'bullet' style projectile you get round the accuracy and range problems, not to mention being able to get the bullet to supersonic speeds without blowing it'self apart like a fart in a hurricane as a plasma would.
Only when the needle is just about to hit the target would you send the laser pulse to vaporise it, forming a small but dense cloud of fast moving metallic plasma.

It's a really elegant way of killing both birds with one superheated needle

You're right, assuming that the atmosphere is similar to Earth's then the plasma would emit the same colour as arc discharges - i.e. purpley blue, just like lightning.

As for reasons why a laser might appear red even without a high Hydrogen, Helium or Neon atmosphere.

If there is dust in the air then that can make lightning look red - One storm I saw moving through a valley in Spain looked like it had cherry red strikes for pretty much the same reason a desert sunset looks red.


Or maybe the wavelength of the laser is near the red end of the spectrum and it's being scattered by smoke, it's own plasma (thermal blooming again) or water vapor, a bit like you'd see in a laser show at a concert.


Where it came from?
I dunno, probably something to do with early sci fi films using the colour red to suggest heat, burning or flames to the audience?

Hmmm - a bullet could get pretty hot due to the explosive of the ammunition - but the only way to get the bullet to both survive the initial explosion and liquify in mid-flight at the touch of a laser would be to use a pretty powerful laser: one that could probably ruin a persons day on their own (at which point, why aren't you just pointing the laser at them - if you have a laser that can reduce metal to a gas in a matter of nanoseconds of contact).

You could combine that idea though with a railgun to fire a 'cold' shot - no explosion - just very rapid acceleration (essentially stationary to laser-light, though) - a metal with a very low melting point like Gallium could then be used to craft the slug, which would reach a melting point at only 30 Celcius (more still required to reduce it from a liquid metal into a gaseous state). Or you could even use a metal alloy like Galinstan which has a melting point of only -19 Celcius.

That way, you would have to actually freeze your ammo to keep it from liquifying in your gun - fire it - it will practically melt just from the air friction - tap it with a laser before it reaches target and you would be shooting a Mandiblaster-style plasma weapon using only current Terra tech

The red laser color probably came from the fact that early lasers were simply red.

The plasma is basically ionized matter. The color depends on what is ionized. High amount of nitrogen in any atmosphere will produce effects similar to those in our, that is lightning etc.

Light IS energy. Photons are both a Wave and a Particle all at the same time. A large laser cannon is for all intents and purposes a photon particle cannon.

The density of this plasma is low because a lightning strike is composed of Electrons while a laser is composed of Photons. MUUUCH faster. The plasma will be denser by orders of magnitude. I do not believe I am far off with the "ion shotgun" hypothesis.

Checked the speeds and yes, an Electron (Electricity) moves at maximum 80,467 KM/s in a Lightning Bolt, while a Photon travels at roughly the speed of light 299,792 KM/s, so it would generate roughly 4 TIMES more Plasma from air&dust collisions. This isn't a significant amount, but this Plasma is also traveling 4 times faster and would cumulate additional plasma on its own.

Source: http://www.physicsclassroom.com/class/estatics/Lesson-4/Lightning

I dunno. It still seems like the game would be more fun if the Lascannon didn't 1-shot Marines. 2-shots sure.

Regarding earlier comment of the low RoF on the tabletop:
a): it is a game.
Similarly, a heavy bolter devastator, in the same timeframe, shoots at some orks in poor cover:
between hitting, wounding and 5+ cover, the average outcome is that he kills about 1 Ork..
Pathetic, eh?

b)
Each Lascannon charge pack is good for only one shot before it must be replaced. This means it has a slow rate of fire due to the need to change the pack after each shot, and the packs cannot be recharged like those of a Lasgun. The power of the Lascannon's fire means that the barrel is damaged quickly by the energy discharge but can easily be changed, much like the Lasgun.
http://warhammer40k.wikia.com/wiki/Lascannon
Sounds like a pretty decent reasoning for low RoF.

Hmm..
I presume this "4 times more plasma" is coming from the kinetic energy equation E= 1/2 * m * v² ?
Well, a photon has much, much less mass than an elektron, and is less likely to interact with atoms it passes than an electron.

I'd also think : at this energy level, around the path, pretty much all the air get's turned into plasma anyway. So, the density of the plasma is determined by the air pressure primarly.
(I'm making quite some assumptions here admittedly; gas, thermodynamics and all that were never really my cup of tea. I presume that at those two very high speeds, our plasma simply doesn't have the time to disperse)

Plasma travelling 4 times faster: not really?
The photon pulse is travelling much faster through the air, so the plasma-generating-energy-pulse is travelling much faster, the plasma itself doesn't suddenly achieve light speed, just because a photon caused the plasma.
I don't see why the plasma itself would travel faster; the speed of the plasma would be pretty negligible compared to that of the photons.

I still see the lascannon as a pulse of laser energy, so powerfull it vaporises the first solid surface it encounters.
The plasma is mostly a byproduct, although one that could be unpleasant to the receiver of the lascannon bolt.
In order for plasma to damage a tank, it needs to deliver a pretty high concentration of energy.
Merely "plasma-ifying" the air is not enough to do that: otherwise cars would be pretty screwed when hit by lighting. Likewise, we have people survive getting struck by lightning. Air, even when turned into plasma, simply is not dense enough to vaporize you.

This actually seems very reasonable based on the same timeframe of roughly 40-45 seconds, considering those Orks are in partial cover, probably dodging in an out, plus factoring overheating and cooldown, even knockback and dodging the incoming fire from the Orks. Actually killing 1 or 2 of the Orks would be pretty good, as the Orks would again have the Cover to protect them.

This would be a drastically different scenario in the open with no cover. I would expect 3-5 kills. Can you be so kind as to do the calculations and get back to us?

I am making a GREAT deal of assumptions. I know very little about particle physics as well. Photons are actually Massless, I do not think Mass has any relation to this calculation. Remember that Photons can use calculations based on both Particle AND Wave physics.

I am sourcing my guess based on the extremely high powered laser example shown at the end of one of my links, and the description of the effects that follow: http://what-if.xkcd.com/13/

"The most powerful laser on Earth is the confinement beam at the National Ignition Facility, a fusion research laboratory. It’s an ultraviolet laser with an output of 500 terawatts. However, it only fires in single pulses lasting a few nanoseconds, so the total energy delivered is about equivalent to a quarter-cup of gasoline.

Let’s imagine we somehow found a way to power and fire it continuously, gave one to everyone, and pointed them all at the Moon. Unfortunately, the laser energy flow would turn the atmosphere to plasma, instantly igniting the Earth’s surface and killing us all."

You make a very good point, however there are three good reasons for converting a laser to plasma; reflection, ablation and refraction.

For instance if the target was reflective (Oooo shiny Reflective armour) that would seriously mess with the laser.
Rather than delivering a fatal, bone deep, tan most of the energy would get bounced off like some burny discoball.


If, on the other hand, the target's outer layer of armour boiled away into an opaque cloud (Ablation) just as the laser hit it then most of the laser's energy would be absorbed by that cloud and harmlessly dispersed away from the target.


Finally a more advanced technology might use metamaterials as a kind of advanced stealth system, the Tau come to mind.
In a metamaterial stealth system the surface of the target is specially engineered to have a negative Refractive index, effectively causing light (and hence lasers) to bend around it's surface. Basically when you look at it you only see what is behind it... Sounds like something out of Harry Potter I know, but we're already working on exactly this.


By converting the laser energy into plasma you make sure that most of it's energy will be delivered in a way that isn't so easily countered - a highly charged fast moving glob of ridiculously hot metal vapour.