I like the idea of laser-guided lightning, in large part because "laser-guided lightning" sounds so fun, but in larger part because I was introduced in early childhood to the concept of the electrolaser and its use for a Taser's electrodes, so it has a lot of sentimental value. Also, the plasma is just a side-effect; air is not a superconductor, so it heats up when you force current through it. Calling a lightning projector a plasma weapon is kinda like running some one over with a truck and calling that truck an acoustic weapon because it makes enough noise to be potentially unpleasant. The directed-energy weapons in battletech are all VASTLY more powerful and sophisticated than would make sense for laser-guided lightning, anyway. Much more importantly, lightning by itself is a pitiful weapon and wouldn't be of concern to even today's armored combat whamdoodles if they were designed with it in mind. Modern aircraft are designed to safely handle being hit by lightning, which they do all the time. I'm pretty sure we can safely assume it is not a threat to a Battlemech.
PPC are fluffed as producing "man-made lightning," but to my knowledge it is not explicitly stated that this is the source of their threat value; quite the contrary, in fact, they are explicitly described as "kinetic weapons" that are effective because they deliver lots of kinetic energy, secondary radiation, explosive effect, thermal stress, and even an EMP effect. Here, the lightning would be to the PPC's main damaging phenomena more or less what the plasma produced by lightning is to the thermal and electrical effects the discharge can deliver; vanishingly small by comparison. It would be moderately threatening to unprotected infantry, though- or rather, would if not for the other effects.
To understand what this means, you need to understand what a death ray is in the first place, and I'll start with lasers since they are probably the most familiar type for most people and because I have a very handy link if you want to read in more depth than my shallow overview offers:
http://panoptesv.com...y/DeathRay.html
First, laser death rays capable of competing with guns are a stupendously amazing engineering feat. So, you wanna destroy that 'mech with thermal strain? Well, for a very good laser, about 10% of your input energy becomes output laser light and the other 90% becomes waste heat for you to deal with- and that's not including whatever waste heat your power source generates, or from the likely specialised power supply hardware you'd probably need, (to store up energy until you have enough for an attack) or the very powerful and focused active cooling you would need to keep the gain medium from getting all cooked like your target. (Active cooling moves heat from one place to another, but adds some heat of its own since it is physically impossible to do any work without generating waste heat.)
The most promising way to shoot stuff to death with light seems to be with a type of death ray we call a "blaster," because it concentrates the laser pulse into such a small space and time on the target that it heats where it hits enough for it to instantly explode. You use a series of hundreds of these pulses, so concentrated that the whole series takes only a fraction of a second, so you have a chance of putting most of them in the same place. The idea is to blast a little bit off, give just enough time for the blasted bit to get out of your way, then blast some more out the bottom of that crater, and repeat until you've bored deep enough to hopefully break something important under the armor. This pausing between pulses is required because all of the light frequencies that we might possibly be able to make lasers for can either go through air but not plasma, or vice-versa. As such, the little bits you are removing become perfect armor for your laser since turning them into puffs of plasma is a side effect of this method of drilling through armor. (or flesh, if that's your thing) If your laser could just shine through that plasma anyway, you would have to defeat all of the air between you and your target since air is opaque to most of the light that shines through plasma. The light that can go through both is called Gamma radiation, and would make for a stupendous weapon except for the unfortunate fact that we can't focus it. Maybe future discoveries in physics can show it to be at least theoretically possible, but it still won't stay focused; when going through anything other than vacuum, it scatters fast. The most common interpretation of Battletech lasers, at least for physics geeks, is a vacuum UV or similar laser so that we can have separate "standard" and "pulse" lasers, which requires that the weapon itself probably has to deal with the beam at or near the intensity that is used to drill into the target; if you use a beam that's wide at the emitter and focused down to a point on-target, you have to defeat several times more air to reach your target since air is opaque to these frequencies. Fortunately, there's actually a "have your cake and eat it too" mechanic called filamentation. I'll leave it to the death ray specialist to explain that one:
http://panoptesv.com.../SelfFocus.html
The TL:DR of filamentation is that EM radiation affects matter since matter is made of stuff that interacts with EM force, and an intense enough light changes the speed at which light travels through air. It just happens, luckily, that it changes in such a way as to literally create a fibre-optic cable out of thin air, so if you focus your laser to a high enough spatial power density, it starts keeping itself focused until it loses enough power (from interacting with air) to no longer be capable of this self-focusing. These filaments are needle-thin, which is perfect for efficiency since you have less volume of air to get through. The kind of power level required is good for making a really big blast; the light itself wouldn't hit very hard, but the violently exploding plasma and hunks of blasted-off armor would deliver a lot of momentum, and the plasma would be moving fast enough to produce a nice EMP- which may or may not be important. Of course, there would also be a lot of heat, which also may or may not be important. Personally, I like to imagine that both are important to a Battlemech, but that should be left for individuals to decide based on "rule of cool." Okay, so now you have a rough idea of what a laser death ray would do at interstellar travel tech level, next is the other broad class of death ray, and my favourite: matter beams!
The basic idea is, you push bits of matter at your target fast enough to be dangerous. Where this differs from ordinary bullet guns is that the projected particles are much smaller and faster. They are hurled at relativistic velocities, which means "very near the speed of light." It also means that these projectiles behave more like light than bullets in most ways. Classical Newtonian mechanics are not perfectly accurate, but close enough for our purposes and probably easier to understand, so I'll explain in these terms and then the numerical difference between this and the more accurate Einsteinian model. First, momentum is a measure of mass and relative velocity. Momentum is always generated in such a way that all of it balances out, and this is a physical law; pushing your rocket exhaust in one way pushes you in the opposite way, and pushing your bullet one way creates recoil pushing in the opposite way. The standard symbol for momentum is P, for mass is M and for velocity is V, so:
P = M*V
Now, kinetic energy is a measure of mass times the square of velocity, or:
E = M*V*V
As you can see, then, kinetic energy is a measure of momentum times velocity. Now, as you probably know, "very near the speed of light" is a pretty high range for velocity, so our matter beam weapon will enjoy very low recoil in proportion to the energy it delivers to the target. In fact, it will be less than a hundred times that of a laser of same output power, unless you get way closer to the speed of light than there's any reason to bother doing. Now, as for the inaccuracy of this physical model: the actual momentum as per Einstein's model is going to be the momentum under Newton's model, but multiplied by a variable called the Lorentz Factor, whose symbol is Gamma. This is:
(1-B^2)^(-1/2) where B is the magnitude of velocity in units of C (as in, "speed of light" C)
In as close to plain english as I know how to put it, you divide your projectile velocity by the speed of light, in the same units of speed of course, then square the result, then subtract this square from one. You then take the square root of this difference and divide one by this square root to get your Lorentz Factor. It probably looks all weird and random, but I see "one over the square root of [whatever]" so often in so many subjects, I am amazed scientific calculators don't have a "to the power of negative one half" button to simplify that. I don't expect you to be looking up the speed of light in your preferred units and evaluating Gamma for a bunch of sample values, so here's a couple I remember from a table some one else computed for me: for 99% of C, Gamma is a bit less than 8. For 99.9% of C, it is a bit less than 24. As you can see, you can get "close enough" to the speed of light, and you'll have a whopping 24 times the recoil of a laser that outputs about the same energy. So, what happens when these very light and now light-like projectiles slam into the target? Well, it's pretty much the same as when light does; they transfer their kinetic energy to the target, which is mostly converted into heat. As with lasers, for a matter beam death ray this heat is most likely going to be used to heat up the target and create an exploding mess of plasma with all the same fun stuff like pushing the target and EMP effects. As you can see, a _properly futuristic_ death ray is not just a thermal weapon, but also a plasma weapon and an EMP weapon. This is why I find ray guns more appealing than bullet guns, at least in a far-future setting with things like Battletech dropships capable of throwing around enough energy to make re-useable thrust bricks a viable alternative to more sensible spaceplanes.
Okay, how about the kind of "plasma weapon" that is not also a beam weapon? First, the functional distinction between a "particle weapon" and a "plasma weapon" is whether the particles have enough time to interact with each other enough to behave more like recognisable plasma than a spray of non-interacting particles. In other words, a plasma weapon is more like a bullet gun such as Battletech's autocannon and Gauss Rifles than it is like a PPC. Strictly speaking, we already have plasma weapons today. They are called flamethrowers, were invented in Ancient Greece IIRC, and recently banned by international law because burning to death is just an ineffably, stupendously horrible way to die. In battletech, there are two different weapons called "flamers." One consumes fuel and does yucky stuff to targets, the other supposedly produces a similar effect by using the stupendous energy available to a battlemech to superheat air and/or engine fuel and spray that at a target. Both types of Flamer have very short range, since trying to spray plasma far would be even harder than trying to spray steam very far, and letting the fuel-consuming one spray burning liquid instead would make the more futuristic Flamer useless by comparison. The fuel-consuming one is not terribly much different from the Battletech Plasma Cannon and Plasma Rifle, though. A Plasma Rifle's ammunition is a special plastic, which the weapon heats all the way to a plasma state, or at least until a lot of it is that hot, then throws it at the target. I like to imagine it all being one simple step; zap the fuel, and the resulting expansion blows it all out the muzzle. The Clan counterpart, the Plasma Cannon, is pretty much the same except that for some reason the Inner Sphere Rifle deals mostly normal damage with some heat, while the Clan Cannon just deals lots of heat. There's also a Battletech infantry weapon called the Needler, which the Plasma Rifle seems to be based on. It is pretty much the same idea; flamethrower-type thing using a futuristic energy source to heat plastic ammo, except that the needler shoots a big mess of sharp flechettes instead of a giant flaming bullet; kinda like a shotgun that shoots nails made of incendiary agent instead of balls of metal. Since laser pistols and "rifles" are common infantry weapons in Battletech, why they can't make a heat ray that would be much more practical than a Needler is a mystery.
In case you've been... conditioned by poop culture "science fiction" which depicts plasma weapons as firing magnetically contained pellets, that's actually less practical than shooting lumps of steam, with enough per shot to scald some one, contained by the same sort of wave that contains a smoke ring; if you can produce such a wave powerful enough to carry that much steam fast enough to get to your target before it cools, the steam would be pointless since the wave would pulverise your target anyway and the excessively heavy steam payload would just make it less efficient. Plasma follows the same rules for pressure, volume and density as gas does, specifically a very buoyant one. It also does not have an unusual specific heat, so the amount needed to heat something appreciably either occupies so much volume you're not going to deliver it all to the target in one moment, (think candle flame or blowtorch, or greek fire or napalm) or is under such pressure that the main weapon effect should be explosive rather than incendiary. (think high explosives) The analogy is not perfect, though, as magnetic confinement of plasma is very difficult. Like balancing two toothpicks stacked on their ends, on top of your finger, there is mathematically a balanced state, but the system has a very strong tendency to evolve away from that state.
Edit: added clarification, fixed use of a wrong word, fought epic battle with gods of parsing and formatting
Edited by Owl Cutter, 19 May 2012 - 05:56 AM.
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