Measurement of the effect of collimator generated wakefields on the beams in the SLC

Collimators with adjustable jaws are used in the SLC linac, arcs, and final focus to eliminate the tails of the beams that produce backgrounds in the SLD detectors. However, if the beams are not centered within the jaws of the collimators, transverse wakefields are generated which act to increase the beam emittances. The sensitivity to the beam offset is largest in the vertical collimators in the linac where the small beam sizes (<100 μm) require that the gap between the jaws be reduced to about a millimeter for effective background reduction. To study the wakefield effect of the collimators, measurements of the induced mean angular kick were made as function of the beam offset in one of the SLC collimators. We are interested in both the linear (dipole) behavior near the axis of the collimator and the nonlinear behavior near the jaws of the collimator. In this paper we present the results together with a comparison to theoretical predictions. Besides helping us to quantify the effect of the collimators in the SLC these results are also useful in understanding their effect in future linear colliders, in which collimators will also be important components