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I'm interested in space warfare from a hard sci-fi perspective. Usually I ask these kind of questions on Worldbuilding SE, but I felt that this one fits this SE better.

Recently read this post on the ToughSF blog about how to counter laser weapons by cooling your graphite armor so it stays cooler than 3500 K. The author says that this will prevent the laser from penetrating the armor. Does this work against both continuous beam and pulsed lasers, only one of them or are the assumptions in the article fundamentally flawed?

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It is pretty easy for a focused, pulsed, laser beam to vaporize a metal surface MUCH faster than any reasonable cooling system could remove the heat. A graphite coated surface would absorb light more easily than a typical metal surface, so would vaporize even more easily.

For a high power continuous (not pulsed) beam, a better defense might be to make the surface of the armor very highly reflective. 99.5% reflectivity is achievable. Maybe even better, the armor could include a retroreflector. That would capture ~95% of the incident light and return it to the source laser. When a laser's beam is reflected back into the laser, it degrades the laser's performance dramatically.

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  • $\begingroup$ I agree with this. For example, consider industrial laser beam cutting/welding. The focused laser beam can cut through several millimeters of steel plate with no problem. $\endgroup$ – Time4Tea Sep 25 '19 at 23:41
  • $\begingroup$ Would the retroreflector layer benefit from beeing cooled? After all, if even 0.5 % of a 100 megawatt lasers energy get absorbed the mirror gets 500000 watts. This will definitely worsen its reflection rate. $\endgroup$ – TheDyingOfLight Sep 26 '19 at 0:01
  • $\begingroup$ "Active cooling" could be something like spraying the back of the retroreflector with water, or even with liquid helium, but a laser pulse with enough power density would heat the front surface and vaporize it before the heat could travel to the back surface. $\endgroup$ – S. McGrew Sep 26 '19 at 0:19
  • $\begingroup$ A high power CO2 laser (far infra red) often has a copper mirror. A special grade of oxygen free high conductivity copper is used because electrical conductivity also means thermal conductivity. Even so, it is sometimes water cooled. The mirror is where the beam is most spread, least focused. If the beam was focused on the mirror, it would destroy the mirror. The smallest amount of dirt on the mirror absorbs heat enough to deform the mirror and degrade the beam. $\endgroup$ – mmesser314 Sep 26 '19 at 1:45

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