A proposal to use microwave quadrupoles to shorten the beam delivery section of CLIC

The chromatic correction of the final focus, based on sextupoles and dipoles, requires for CLIC at 3 TeV a section which is long with respect to the main linac and the bare de-magnification telescope. The length needed is in conflict with the tight alignment of the beam delivery elements, necessary for the control of the collisions. This scheme also implies large peaks of betatron amplitudes which generate aberrations. To circumvent these problems, we explore the potential of microwave quadrupoles. They could be used for chromatic corrections since, in the presence of a correlation between momentum and position within the bunch, they play a role similar to that of sextupoles in the presence of dispersion. The correlation is done in the linac and no special optics is required, thus strongly reducing the space needed for the correction. Furthermore, the final doublet of the telescope could be only made of microwave quadrupoles, provided sufficiently high gradients be achievable, since a judicious choice of the RF phase render them achromatic. This would make the beam delivery at high energies even more compact and simple. We underline advantages and drawbacks of this proposal and list some items which need further study.