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 ver- tical deflection of the beam is measured on 29 downstream beam position monitors. A series of apertures with dif- ferent geometries were studied to permit confirmation of the scaling laws and magnitude of deflection predicted by theory. Description of the apparatus and collimator aper- ture specifications, predicted centroid deflections, and mea- sured deflections are discussed.