Very High Power Light Beams Consider a collimated light beam in a vacuum (I am being unspecific about the frequency, anything from radio to gamma). If the beam power/cross section was increased indefinitely would new phenomena occur at certain energy densities?
I gather that pair-production couldn't occur due to momentum considerations but would there be some interaction with the quantum vacuum? 
At sufficiently high power is there a self-gravitation effect of the beam, a point touched on by a previous question? What if the beam was reflected back on itself by a mirror?
Note: the motivation for this question relates to theoretical limits for beam weapons.
 A: You have some information about photon-photon scattering in vacuum on the Extreme-Light Infrastructure's website. In particular, they say 

For electric fields $E\sim mc^2/λ_C ~ 10^{29} {\mathrm W}/{\mathrm cm}^2$, where $λ_C$ is the Compton wavelength (Schwinger (1951)), virtual electron-positron pairs will be able to separate and become real.

Which means that various interesting effects happens here, even in vacuum, limiting the power of the light beam. 
While ELI's website shows that it these effects will probably seen in not too futuristic experiment, this density is expected to be reached in very short (attosecond) pulses at a low repetition rate. For a weapon, the relevant quantity is the average power, and making
a weapon which concentrates the power of $10^{20}$ nuclear reactor in one  cm² doesn't seam very realistic, except if you build a Death Star ;-)
A: OK Georg, I am replying to this because it is coming around once more, and good answers exist in wikipedia. as I am too rusty to sit and calculate.

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. Blooming can be more severe if there is fog, smoke, or dust in the air.

So the effect of strong energy in a laser beam ( the one known coherent electromagnetic weapon grade) turns air into plasma after a certain energy density. It will melt whatever it hits at those energies, which after all is the purpose of the weapon. Mirrors are out because the reflecting surface absorb part of the radiation and will be destroyed at such energy densities. 
In principle a coherent high energy density beam of soft photons could also pair produce by higher order diagrams, but one would have to calculate at what energy density this would destroy the beam even in vacuum.
If the question is really "how to defend a target from a laser weapon", the answer is "smoke", aluminum confetti and such.
