Ultimate parameters of the photon collider at the ILC ∗

At linear colliders, the e + e − luminosity is limited by beam-collision effects, which determine the required emittances of beams in damping rings (DRs). While in γγ collisions at the photon collider, these effects are absent, and so smaller emittances are desirable. In present damping rings designs, nominal DR parameters correspond to those required for e + e − collisions. In this note, I would like to stress once again that as soon as we plan the photon-collider mode of ILC operation, the damping-ring emittances are dictated by the photon-collider requirements—namely, they should be as small as possible. This can be achieved by adding more wigglers to the DRs; the incremental cost is easily justified by a considerable potential improvement of the γγ luminosity. No expert analysis exists as of yet, but it seems realistic to obtain a factor five increase of the γγ luminosity compared to the " nominal " DR design.