Polarization Issues at CLIC

We review polarization issues for CLIC at 3 TeV centre-of-mass energy. An electron beam with about 80% polarization can be produced by an SLC-type photo-injector. Compton scattering off a high-power laser beam may provide a source of positrons with 60%-80% polarization. If the spin transport is taken into account in the geometric layout of the facility and in the choice of local beam energy, no significant depolarization is expected to occur on the way to the collision point. We demonstrate this explicitly by spin tracking through the beam delivery system. During the beam-beam collision itself, due to beamstrahlung and the strong fields at 3 TeV, about 7% of effective polarization will be lost. A polarimeter for the spent beam appears indispensable.