[Kosh]

Does anyone have any theories for how they work?

[The E]

Pretty well, especially against fighter shields 

That being said, we know a piece of phlebotinum called a "Plasma core" is involved in its operation, which needs to be inserted before firing. Their power output can be increased by overloading them, although this is definitely dangerous and can lead to secondary damage on the firing ship.
As for actual technobabble, you are free to create your own.

[Aardwolf]

"Power up photon beam cannons", "Commence plasma core insertion".

[Asteroth]

As evidenced by Aaradwolf they seem to be based on photons, and ineed high enough evergy photons would heat up it's target, but the beam certainly wouldn't be visible and the range would be extremely long. The largest range problems for lasers (just a beam of photons) is diffusion, because of the air particles the beam spreads out after very long distances (If i remeber correctly I think they're good up to like 10-20-30 miles), this problem is of course not quite as relavent cue to the distinct lack of air, thusly the range could be up to interplanetary. The actual amount of damage am am unsure of.

[Droid803]

They've also been referred to as "Flux Cannons". Well, only the Lucifer's guns, but I'd presume the GTVA and Shivan instances are based off the same principle.

[NGTM-1R]

Electrolasers, accounting for their shield-puncturing (we know shields don't like raw EM in large doses) and other behavior.

I've also posisted the existence of beam-like weapons that stream magentically bottled plasma or antimatter.

[General Battuta]

Electrolasers

I think there's a key ingredient of the electrolaser missing, my friend.

Unless, of course, the weapon first deploys a particle medium...but that seems kind of roundabout.

[NGTM-1R]

Yes, well, it's not going to work, but neither is any other explanation, so I'm in good company.

[Mongoose]

Despite the "photon beam" name, I've always felt like they made more sense as some sort of particle plasma cannons, especially with the "plasma core insertion" line.  If one were able to generate a powerful bound magnetic field, one would presumably be able to accelerate charged particles at a decent fraction of c along it, which would probably deliver a massive punch on the other end.  The shield-piercing behavior of AAA beams could be explained away by said incredibly high-energy ions completely overwhelming shield defenses over a focused area, to the point where they impact the hull mostly unhindered.

[Asakura]

As evidenced by Aaradwolf they seem to be based on photons, and ineed high enough evergy photons would heat up it's target, but the beam certainly wouldn't be visible and the range would be extremely long. The largest range problems for lasers (just a beam of photons) is diffusion, because of the air particles the beam spreads out after very long distances (If i remeber correctly I think they're good up to like 10-20-30 miles), this problem is of course not quite as relavent cue to the distinct lack of air, thusly the range could be up to interplanetary. The actual amount of damage am am unsure of.

The process is called attenuation. It can happen even in space, but the effect is pretty much negligible over distances of the mission area. Just imagine it as an oversized laser, and whether or not the actual beam will be visible should such a thing exist would depend on a lot of things, such as the wavelength and intensity of the beam used.

[deathfun]

You folks and your knowledge over this matter

I just know not to be in the beams path when it fires. Too many times have I been massacred for being in the wrong place, at the wrong time

[Asakura]

I just died 5 times in Rebels & Renegades.

1st was just getting ripped apart by the Sobek's AAAf.
2nd was getting caught by the Sobek's VSlash.
3rd was getting the Iceni's BGreen jammed in my face.
The last two times were courtesy of the Iceni's AAAf while I chased the fighters around it.

Based on a 60% death by friendly fire, I would presume that the NTF already knew I was an agent before the Sunder.

[deathfun]

I had the unlucky opportunity to get in the way of the Juggernaut vs the Collie

The Collie wasted me and the Juggernaut took my remains

[Liberator]

The way I would think it would would would be that during the "wind up phase" where the capacitors are whining as they absorb the charge to fire, the beam cannon is already firing some sort of trace beam which is much lower power than the actual weapon and is also the opposite polarity of the beam particles, so that when the capacitance system reaches maximum saturation and discharges the main particle beam remains focused in a fairly tight beam instead of being like a flash light and spreading out at a wide degree to the actual target.

[Black Wolf]

Well, I think I have a workable theory. First, a "core" of beam material is inserted into a chamber (the "plasma core") - probaby some kind of high atomic weight noble gas, Krypton or Xenon - since we know the GTVA has used them in weapons before. Next, this core is excited by extremely high energy photons up to extremely high temperatures, stripping away the electrons and converting it to a charged plasma. Thirdly, a magnetic control "tube" is initiated to keep the plasma in a coherent beam shape. Finally, a small hole opens in the chamber, aligned with the magnetic controlled "tube", and the extreme pressure of the plasma (plus possibly a degree of magnetic acceleration) forces the particles out along the predetermined path, where they smash into the enemy ship.

The charge glow might be explainable by slightly less than completely airtight seal allowing a small amount of plasma to escape, cuasing the glow around the firing point as they spread out in all directions.

[Kosh]

Thirdly, a magnetic control "tube" is initiated to keep the plasma in a coherent beam shape.

Based on our known physics today, is that even possible?

[Asakura]

You know, there is a reason why it is called science fiction.

[Stormkeeper]

Or the 'plasma core' referenced could simply be a cartridge containing the energy required for a full blast. It would help alleviate the draw on the ship's reactor that way.

Also,

You know, there is a reason why it is called science fiction.

QFT.

[nvsblmnc]

The way I see it, there are 3 distinct types in use by the GTVA.  My decidedly non-canon interpretation:

Green: Terran beam weapons which work by bleeding off plasma from the ship's reactor.  These have the advantages of being relativley small, allowing a given vessel to carry more of them, and they can be 'overcharged' by forcing greater amounts of plamsa through the emitters.  This comes with the cost of increased thermal load on the cannons, and can also damage the reactors themselves if too much plasma is drained.

Yellow: Vasudan beam weapons which utilise directed photon beams.  These are more powerful than the green Terran beams, but require a dedicated reactor to power them.  As a result of this increased bulk, the weapons are typcially mounted in smaller numbers, and not at all on cruisers.  We've not seen if these can be 'overcharged'.

Blue: Ion beams shared by both races.  These are very energy efficient and have a low thermal load on the emitters, allowing for rapid refiring and continual operation, but do very little damage.  Because they can draw power from the carrying vessel's standard energy grid, practically any vessel of cruiser tonnage or above can mount worrying numbers of these weapons.

As for the Shivans, I like to think of their beams as being some kind of graser, if only due to rule of cool.

[Snail]

Well, I think I have a workable theory. First, a "core" of beam material is inserted into a chamber (the "plasma core") - probaby some kind of high atomic weight noble gas, Krypton or Xenon - since we know the GTVA has used them in weapons before. Next, this core is excited by extremely high energy photons up to extremely high temperatures, stripping away the electrons and converting it to a charged plasma. Thirdly, a magnetic control "tube" is initiated to keep the plasma in a coherent beam shape. Finally, a small hole opens in the chamber, aligned with the magnetic controlled "tube", and the extreme pressure of the plasma (plus possibly a degree of magnetic acceleration) forces the particles out along the predetermined path, where they smash into the enemy ship.

The charge glow might be explainable by slightly less than completely airtight seal allowing a small amount of plasma to escape, cuasing the glow around the firing point as they spread out in all directions.

I like this one the best.

Find a way to get the word Flux in there (maybe Flux is the amount of charged plasma being vented?) and I'll officially declare it interesting.