How to Build a Laser Death Ray
As we learned in the section on linear absorption and scattering, light going through the air can be absorbed. Even if only a small portion of the light is absorbed, high power beams can still significantly heat the air. Hot air will expand so that it reaches a lower density, leading to a core of lower density air through which the beam passes with higher density air outside. The gradient in air density acts like a weak lens that acts to defocus the beam. This defocusing due to heated air is called thermal blooming.
If the beam is tracking a moving target, or if the wind is blowing cooler air across the beam, you end up with the hot air mostly on one side of the beam. This makes the air act as a lens to bend the beam toward the cooler, denser air, which will throw off your aim.
Thermal blooming can be difficult to adaptively correct. An adaptive optics system will tend to try to compensate for the increased divergence of the beam by focusing the beam even more. This makes the light more concentrated, which heats the air even more, which makes the thermal blooming even worse! Adaptive optics systems may be able to be designed to handle blooming - or they may not, we do not yet know.
There are ways to avoid thermal blooming. The most straightforwad is to use short, high power pulses. Hot air takes time to expand into low density air, so if the beam pulse is done propagating before the air expands, it will not be affected. The time it takes before the hot air expands depends on the beam width, with wider beams having more time before the onset of blooming. Meter wide beams do not suffer significant blooming for perhaps a couple of hudredths of a second, centimeter wide beams for perhaps a few tens of microseconds. This means that practical death rays that are pulsed for a few tens of microseconds or less need not concern themselves with thermal blooming.
Even if you want to put a continuous ray of heat on the target, you can mitigate the effects of thermal blooming. The closer the range, the less the blooming. The less your wavelength of light is absorbed, the less the blooming. The wider your beam, the less blooming it causes. The faster the wind (or the faster your beam slews to follow a target) the less blooming the beam will experience. Finally, it is obvious that the less powerful the beam the less blooming you will experience, but as death ray designers we do not want to go too far in this direction!