Sponge: This sort of thing could be used for a focusing element on a large x-ray laser turret, though the material you'd make it out of would probably have to be a large very precisely machined crystal. Additionally, x-ray mirrors
exist, so I'm not sure what the cutoff energy level is that Campbell was talking about. I'm not an engineer so I have no real information to give you about it. Which is why I'm getting on my
research pants! Cruising on down
here on the european XFEL site, it seems they're using... nothing at all to focus their beams beyond the electrons in the undulator. Since they don't need to focus their beam very much and X-rays are so bloody short wavelength they don't attenuate very much, this works for their purposes.
Let's talk about diffraction for a moment; assuming that its possible to machine a nanoscale diffraction element; in order to bend the beam you'd need to be making grooves on the same scale as the wavelengths which would require TRICKSY engineers. Thinking about it, you MIGHT be able to get away with a flexible lens changing spot size, but you might not. This would make death-star type lasers need to be built for a single desired focus and range. Outside that range, they'd quickly lose focus and damaging capabilities.
However, there would be no moving components in this turret, so the death star would likely have a number of different turrets the electron beam generated in its guts would be routed to, each with a different focus. Super long rage "interplanetary graffiti" turret, a number of .1 LS turrets for missile interception, and probably one for light second range "maybe we'll get lucky" shots. Similarly, if you wanted to invest in a number of undulators, you could split the electron beam and route it through multiple point defense turret, allowing you to very nicely combine and disperse laser power.
The kilometer accelerator ring is a valid point; you'll note that earlier I proposed that sort of weapon on star destroyers and battlestars for just that reason.
This is a fun little application to use for calculating laser coolant capacity. Being a sheep I'm using lithium like they use on atomic rocket, and assume the death star can have a 100 ton block of lithium as its coolant. Heavy, yes, but keep in mind that the Saturn V is 30 times more massive than that. Assuming 10 MW shots lasting 1.25 seconds we can try and calculate waste heat per shot: Wh = Tf * (Bp/eff * dc) * (1 - eff), where Wh = waste heat, Tf = firing time, Bp = aperture power, eff = efficiency, and dc = duty cycle.
Tf = 1.25 s, Bp = 10 MW, eff = 10% (.1), dc = oh hell, call it 2 seconds. Wh = 220.5 MWseconds; I'm not sure I picked anywhere near an appropriate duty cycle considering the accelerator will first have to be spun up. This will probably require a great BIG radiator all of its own.
Anyway, we have waste heat values. Let's plug them into that coolant calculator I linked just a bit ago. Running the numbers, it gives 667 activations, for a continuous activation time of 1400 seconds, over 20 minutes, per 100t chunk of lithium. Plugging in
433 Eros like I suggested as a base for such an orbital installation gives access to a 6.69E15 kg chunk of silicate rock. I'm gonna use water as a representative sample of that; it's programmed into the calculator and I'm having a bitch of a time looking up applicable feldspars to use as a substitute. As 1000 tons of water gives 1830 activations and an hour of firing, I'm going to assume that 6690 trillion tons of water will give 6.69 trillion hours of firing, or 763 million years of continuous firing.
This means that the accelerators for the laser(s) could be reasonably left running indefinately, giving the time required for the death star to react to a detected threat to be on the order of the time it takes a controller to click the "zap that shit" icon on his desktop. Fearsome indeed.
Now, that's assuming relatively small amounts of warm up time per shot and 10% efficiency for the entire system. But -still-, that kind of massive thermal reservoir would completely alleviate all needs for thermal systems. Assuming you placed the thing in an orbit where it had a great view of the earth area at all times and drilled through from pole to pole for maximum coverage you could construct many and larger death ray lasers on Eros, turning it into a relatively unassailable object. A number of such orbital fortifications built out of redirected apollo-amor asteroids would actually make things quite safe from whatever.
Cost of such a thing would be substantial; smaller rocks would do just fine and be easier to redirect though many of the A-A asteroids are quite close in terms of required delta-V (sneer sneer) to get them into an earth orbit. By no means would the rock hold all the requisite materials either; you'd need to manufacture a fairly significant amount of electronics and probably accelerator elements either on the moon or Earth; boosting that would be a royal bitch. Would such a massive endeavor be as expensive as a fleet of killships with missile swarms? I'm almost entirely positive it'd be far more expensive, but it'd also be an investment that was a lot sturdier. Teraton rocks would require enormous energy to pulverise from missile strikes; impact shocks would probably destroy any installations far before the missiles even scratched the thing. Massive kinetic impact (read: another rock) would probably be a better bet, but such a thing would have to take a ballistic course to the target over a prodigious distance and thus the death rays woud be able to engage.
Such an engagement would be quite tactical; while the laser's impact would produce thrust on the impactor, should the attackers have the asteroid suitably de-spun they would be able to attach a rocket with similar output to the laser on the opposite side and compensate. Small-spot lasers would take many, many, many consecutive hits to burn through all of that rock and aiming at huge range would be a bitch and a half. Additionally such a super rock would require the ability to correct course relatively well; anyone going to the trouble of building such a death star would do well to include docking arrangements for tugs capable of moving it out of the way of the impending death projectile. Otherwise a laser fort's only defense would be point blank thermonuclear salvo attempting to deflect the thing (long shot) or a really, really good insurance policy.
Thanto: That was sort of why I plugged the death ray in vs. fullerite (nanotubes), about the badassin'est armor I can imagine. Again, to fight lasers you need armor that can wick away heat DAMN FAST. Vapor does not contrary to popular belief disperse all incident radiation. In fact, molten metal is closer to a black body and thus VERY GOOD at absorbing radiation. In other words, it would have to be vaporized by the beam.
Except, that's how the damn beam drills through armor in the first place; a pulse melts the outermost layer which expands, vaporizes, creates massive localized pressure and fragmentation which drives out particles of armor, and then the melt front propagates through the target. High energy beams pulsing at greater than a MHz will cause the individual shocks of each pulse's explosion to coalesce into a single wave propagating through the hull.
Again, laser impact != reentry. Reentry is friction across the surface of the space shuttle, laser spot = rapid spot heating. Both are deadly and involve large numbers of degrees ganging up to break shit, but the laser does it in a way that is far harder to effectively defend against because 1) it's faster and 2) it's localized.
In conclusion: You don't want Mass Effect armor that vaporizes when it gets hit by a laser as it is doing the laser's job for it. Lasers are not like relativistic impactors, they are not defeated by whipple plating or ablative layers. You'd probably want something with a very large vaporization energy for its mass. You would also want to be in tight control of what aspect you present to the laser; this is why I recommended the orion drive ship attacking with its pusher plate towards the target. The pusher plate needs to be a massive chunk of metal anyway; you're not going to realistically afford similar protection for the front of the ship. If you didn't use an orion drive, you would likely end up with a super-armored ship prow ith whipple plating to deal with kinetic weaponry and dust collisions (nothing to sneer at at 100s of km/sec relative speed), Awesome Shit (carbon or boron are the best current material) to absorb laser energy, and shock absorbers to help mitigate the blast effects of a contact nuclear detonation. For that last one, you will need a very large vessel with lots of armor.
Normandy: Oooh you're getting the yeast in my science loaf to really rise!
Moving away from plagiarism, the interaction with x-rays and atoms is tricky. For while they will hang around depositing energy in dense stuff, they also slam right into the atmosphere. Just cause it isn't dense doesn't mean the material is transparent to that radiation wavelength.
I'm honestly not sure how diffracted the beam would be by such a spongey substance; any energy deposition at all would probably lead to rapid heating or vaporization of the material because it's being hit by a freaking laser and lasers vaporize shit.
For a HS physics student, you're making pretty thoughtful points.
EDIT: On the subject of the immense thermal capacity of 433 Eros: Being a S-type asteroid, aka a carbonaceous chondrite, it has a significant amount of water. Hollow out a large volume in the interior and fill it with water. Submerse the accelerator in this water, then agitate it to spread out heat. To defend against impact shock from surface nuclear detonation, surround this chamber with foam-filled voids. A friend points out that the scree from asteroid mining would work nicely in lieu of foam; all it needs to do is give a little to reduce shock effects.
Defeating such a thing would reduce to destroying the turrets, of which it could have many. Very, very, very, very many. Teraton asteroids are difficult to engage as an entity. Terawatt range lasers would probably be the best bet, as they have beastly cutting power. A friend suggests a barrage of X0 or X00 ton asteroids and assorted shot to attack the turrets directly and attempt to saturate defenses before delivering a coup de grace kinetic impact. Attacking the turrets with your own lasers would work well too.