Lattice scaling and emittance control in the CLIC main linac

For the main linac of the CERN linear collider (CLIC), emittance degradation due to wake fields and misalignments was previously investigated at unvarying injection energy and scaling of the betatron function. In particular, to keep a constant stability margin along the linac, the scaling retained was such that β²/γ was constant. Recent developments suggest that the lattice scaling along the linac should be modulated differently in order to better balance the effects of the wake fields with respect to the chromatic dispersion. This may help to reduce the emittance dilution as would also a reasonable increase of the injection energy compatible with a bunch compressor. In the case of 250-GeV CLIC linacs, it was found that independent scaling of the cell length and focusing strength, and scaling laws different from the one mentioned above allow a better control of the growth of the small emittances foreseen, especially in the vertical plane. This approach, combined with a simultaneous adjustment of the injection energy and an improvement of the trajectory mastering, has made it possible to relax the alignment tolerances on the quadrupoles and the cavities while keeping the final emittances at their required level