Direct Measurement of Geometric Wakefields from Tapered Rectangular Collimators

Direct measurement of the short-range wakefield due to a series of tapered collimators is presented. The wakefields were measured by inserting a fixed aperture in the path of the electron beam at the 1.19 GeV point of the SLAC linac. As the vertical position of the collimator is varied, the vertical deflection of the beam is measured on 29 downstream beam position monitors. A series of apertures with different geometries were studied to permit confirmation of the scaling laws and magnitude of deflection predicted by theory. Description of the apparatus and collimator aperture specifications, predicted centroid deflections, and measured deflections are discussed