  At photon colliders gamma-gamma, gamma-electron high energy photons are
produced by Compton scattering of laser light off the high energy electrons (or
positrons) at a linear collider. At first sight, photon colliders based on e-e-
or e+e- primary beams have similar properties and therefore for convenience one
can use e+e- beams both for e+e- and gamma-gamma modes of operation. Below we
compare these options and show that e-e- beams are much better (mandatory)
because in the e+e- case low energy background gamma-gamma to hadrons is much
higher and e+e- annihilation reactions present a very serious background for
gamma-gamma processes.
