Atmospheric Hole Burning
In the page on attenuation, we said that frequencies higher than ultraviolet-C - this would be vacuum ultraviolet, x-rays, and gamma rays - can’t get through air. This is mostly true, but there might be a way around it. At least for short ranges or tenuous atmospheres. The trick is to use the laser to “burn” a hole in the air for the beam to go through.
Vacuum frequency photons going through air are removed from play by interacting with an air molecule or atom and knocking one of its electrons out, with the photon disappearing in the process. But if previous photons in the beam have already knocked out all the electrons that are bound by less energy than the beam photon energy, there are no more electrons that can be knocked out. So the photon just keeps trucking through the plasma around it. In effect, an intense enough beam of ionizing radiation makes the air transparent by turning it to plasma by the very process of its absorption. The beam photons will eventually be scattered out of the beam by interactions with electrons in the plasma, but this can take several tens of meters for sea level air.
So what do you do if you want to shoot through more than several tens of meters of air? first send out a pulse that ionizes the first, say, 20 meters of air into a star-hot plasma. Then wait just a fraction of a second for that plasma to expand to near vacuum. Then send another pulse through that vacuum channel you just made, which will ionize another 20 meters of air. Repeat as needed.
There are two issues with this method. First, it takes a lot of energy to ionize all that air, energy that you can’t then use to blast holes in your enemies. As a rough rule of thumb, air is something like 1000 times less dense than water (or people). So every meter of air the beam goes through removes a millimeter of penetration into a person, or an eighth of a millimeter of penetration into steel (because steel is approximately 8 times denser than water or people). (More exactly, sea level air is more like 830 times less dense than water, so it’s more like you are losing 1.2 mm of penetration through people per meter of air your beam goes through.)
The other is that the beam is still subject to diffraction. In order to have the beam waste as little energy as possible ionizing all that pesky air in the way, you will want to make the beam really skinny so it has less total air to go through. Like hair-thin. And even though you are shooting really short wavelengths this very narrow beam still makes diffraction occur over annoyingly short distances.
At least it will look awesome. You get a brilliant blue-white streak-flash of plasma like a bolt of straight lightning, producing a dramatic thunderclap to enhance the show.
If you want to work the details out for yourself, it takes about 12.8 kJ to completely ionize a cubic centimeter of sea-level density air.  It also takes x-rays of 871 eV or more (1.42 nm or less) to zap off these most tightly bound core electrons to make the air completely ionized (if you lower the x-ray energy you don’t have to spend as much energy because you can’t fully ionize all of the atoms - but then you are also fighting even worse diffraction). At this point, the fully ionized plasma that used to be air will have a temperature of around 10 million kelvin and a speed of sound of about 90 km/s. The scattering length of x-rays through this plasma (due to a process called Thomson scattering) at sea-level density is 41.6 m.
Author: Luke Campbell
- Calculated using ionization potentials taken from David R. Lide, "CRC Handbook of Chemistry and Physics: 71st Edition 1990-1991", CRC Press (1990)