Slashdot Mirror
Navy's New Laser Weapon: Hype Or Reality?
Lasrick writes: MIT's Subrata Ghoshroy deconstructs the Navy's recent claim of successful testing with the Laser Weapon System. It seems the test videos released to the press in December were nothing more than a dog-and-pony show with scaled-down expectations so as to appear successful: "When they couldn't get a laser lightweight enough to fit on a ship while still being powerful enough to burn through the metal skin of an incoming nuclear missile, they simply changed their goal to something akin to puncturing the side of an Iranian rubber dinghy." Ghoshroy is an entertaining writer and an old hand in the laser research industry. He gives a explanation here of the history of laser weapons, and how the search for combat-ready tech continues: 'At the end of the day, good beam quality and good SWAP—size, weight and power—still determine the success or failure of a given laser weapon, and we're just not anywhere near meeting all those requirements simultaneously.'
Your mirror would cease to be a mirror in very short order by either sheer ablation or the formation of oxides, reducing its ability to reflect, causing the absorption of more energy, at which point your mirror ablates. HTH.
The mirror does not reflect 95% up until it fails and then starts absorbing 95% of the laser energy. Your (let's say) silver mirror would absorb a tiny bit of energy (say 2.5kW), which is enough to start heating up. This accelerates the formation of oxides, and the oxides are nowhere near 95% reflective. Those oxides rapidly absorb the laser energy, ablate, and so on. Obviously having shiny mirrored surfaces help, but not nearly to the extent one would hope.
dom
Where do you see that the mirror would quickly lose the ability to reflect compared to exposure time ?
Because, even assuming your surface remains polished and dirt free so it can have such reflectance, it doesn't take much energy to ablate a microscopic amount of material and greatly reduce the reflectance of the surface. Even if it is solid metal, laser damage tends to produce very absorbing surfaces as opposed to just exposing more reflective surfaces. The amount of energy to degrade a surface is much less than needed to do bulk damage or even bulk heating, and hence tends to be a small fraction of the time you were intending to hit the target for anyway. So your 0.25 doesn't turn into 4 s if the surface reflectivity drops off in less than 0.1s. And any sort of dirt or surface defect can greatly speed up the process, with damage spreading from that point.
This comes from experience working with mirrors purposely built to handle high power lasers, both those that are wavelength specific (and tend to be better than 99.9% reflective) and those that are not. Once your beam becomes a little too focused (e.g. a hot spot from bad beam quality) or you let the mirror become dirty in some way, failure is pretty much instantaneous, leaving a large splotch of stripped reflective coating or roughened metal surface.