  A high energy photon collider (gamma-gamma, gamma-electron) 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: wave
length of the laser should be about 1 micron, i.e. in the region of most
powerful solid state lasers, collision effects do not restrict the gamma-gamma
luminosity. In the multi-TeV energy region the situation is more complicated:
the optimum laser wave length increases in proportionally with 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), this
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 of 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 gamma-gamma luminosities at multi-TeV
photon colliders is suggested.
