Wednesday, March 17, 2010

Blooming - A Problem with Ray Guns

 "Ray guns are in our future, and always will be."


... Skeptical retired U.S. Air Force General, since proven wrong.


Yes, Ray Guns exist. They're being used now to give headaches to suspicious characters approaching checkpoints - sort of a low level "stunner" of sci-fi fame. They're low power infrared lasers with wide dispersion, yes, but they're a start. We're quite a ways from Star Wars type blasters, but we've started down the road.


The dispersion thing is a real drawback, but Science marches on, and these are engineering problems for the present and immediate future and long-term. It's only a matter of time.


Still ...


From the Wikipedia article, "Directed-Energy Weapon":




Laser beams begin to cause plasma breakdown in the air at energy densities of around a megajoule per cubic centimeter. This effect, called "blooming," causes the laser to defocus and disperse energy into the atmosphere. It can be more severe if there is fog, smoke, or dust in the air.
There are several ways to stop or reduce blooming:
  • The beam can be distributed over a large mirror that focuses the power on the target, to keep energy density in the air too low for blooming to happen. This requires a large, very precise, fragile mirror, mounted somewhat like a searchlight, requiring bulky machinery to slew the mirror to aim the laser.
  • A phased array. For the usual laser wavelengths this method would need billions of micrometre-size antennae, and no way to make these is known. Phased arrays could theoretically also perform phase-conjugate amplification (see below). Another advantage is that phased arrays do not require mirrors or lenses, can be made flat and thus do not require a turret-like system (as in the first approach) to be aimed, though range will suffer at extreme angles (that is, the angle the beam forms to the surface of the phased array).[1]
  • A phase-conjugate laser system. Here, a "finder" or "guide" laser illuminates the target. Any mirror-like ("specular") points on the target reflect light that is sensed by the weapon's primary amplifier. The weapon-power amplifier then amplifies inverted waves in a positive feedback loop, destroying the target with shockwaves as the specular regions evaporate. This avoids the blooming problem because the waves from the target passed through the blooming, and therefore show the most conductive optical path; this automatically corrects for the distortions caused by blooming. Experimental systems using this method usually use special chemicals to form a "phase conjugate mirror." In most systems, the mirror overheats dramatically at weaponized power levels.
  • A very short pulse that finishes before blooming interferes.

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