Problems of multi-TeV photon colliders

A high-energy photon collider (γγ,γe) based on backward Compton scattering of laser light is a very natural supplement to e+e− a linear collider and can significantly enrich the physics program. The region below about one 0.5–1 TeV is very convenient from a technical point of view: wavelength of the laser should be about 1 μm, i.e. in the region of most powerful solid-state lasers, collision effects do not restrict the γγ luminosity. In the multi-TeV energy region the situation is more complicated: the optimum laser wavelength increases proportionally to the energy, the required flash energy also increases due to nonlinear effects in the Compton scattering; bunch trains are shorter (for warm high gradient linacs), which leads to higher backgrounds; the collision effects (coherent pair e+e− pair creation) restrict the luminosity. These problems and possible solutions are discussed in this paper. A method of laser focusing is considered which allows the decrease in the required laser flash energy and the practical elimination of the problem of nonlinear effects in Compton scattering; a way to reduce collision effects and obtain ultimate γγ luminosities at multi-TeV photon colliders is suggested.