Re: Laser protection
- From: Luke Campbell <lwcamp@xxxxxxxxx>
- Date: Fri, 8 Feb 2008 09:47:56 -0800 (PST)
On Feb 7, 5:53 pm, MacFrag...@xxxxxxxxxxxxxx wrote:
In a setting where you have to reckon with powerful reusable lasers,
wouldn't it make sense to make a spacecraft's hull highly reflective
-- effectively, a mirror?
In some cases, it could help. Consider two spacecraft armed with
lasers, wanting to duke it out with each other. If one spacecraft can
hover at a range where his lasers can zap the other guy, but where the
other guy's lasers can't zap him, he'll be doing pretty well. Since
this scenario happens at the edge of a laser's useful range, we are
talking long slow exposures where sections of the hull are heated to
incandescence before melting. So imagine if one spacecraft had a
mirrored hull (reflectivity of about 0.98) and the other didn't
(reflectivity of about 0.5). Assuming everything else were equal, the
mirrored spacecraft could get a bit closer and roast his opponent
while still being able to withstand the withering heat of the other
guy's heat rays. There are caveats about this limitation - the higher
your reflectivity, the lower your emissivity, so it will be harder to
dump heat. You could always keep your radiator turned away from your
opponent if you only have one opponent, but this scenario is not
always possible.
If you get much closer than this, however, mirroring does not help.
Intensities get high enough to make the mirrored surface pitted,
blistered, scorched, and cracked, at which point is stops acting like
a good mirror and begins absorbing the beam.
No, I'm not trying to argue that the beam could be returned to
sender ;), I'm just thinking that the beams could be reflected
randomly into space, as not to harm the original target.
Essentially, it boils down to this: the beam has to reflect off
several mirrors before it leaves the cannon, so the wavelength must
permit reflection. So what happens if the laser beam hits another
mirror after leaving the cannon?
While reflecting through the mirrors meant to point the beam, the beam
will be spread out over a wide enough area that it will not damage the
mirrors. The final mirror, however, is curved to focus the beam to a
small spot on the target. This concentrates the light to a higher
intensity than mirrors can handle.
Or will such powerful lasers work without any mirrors at all, and have
beams impossible to reflect?
Such lasers are possible, people are building x-ray free electron
lasers like this right now (I think one is already on line at SLAC,
and one is being built at DESY). These are research lasers, but the
beams are so intense that they instantly vaporize the research targets
(take your data fast!). There are no mirrors, you use one-pass
amplification of the initial synchrotron radiation from the beams. In
fact, the beams never touch matter until they hit the research target.
The research designs are less than optimal for weapons. For one thing
the beams are very narrow, so they suffer from large diffraction
spreading over the distances likely to be found in space combat.
There may be ways around this - one of my favorite designs is to use a
resonant cavity of x-ray diffraction crystals
http://panoptesv.com/SciFi/DiffractiveCavities.html
to create a high quality, focused "seed" beam that is then amplified,
so the high power beam never touches matter but is wide enough to keep
focus over long distances.
Luke
.
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- From: MacFraggin
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