Direct measurement of the transverse wakefields of tapered collimators

We report on a recent set of measurements of the transverse wakefields from longitudinally tapered collimators. The measurements were performed with a low-emittance 1.19 GeV beam in the SLAC linac by inserting a collimator aperture into the beam path and reconstructing the vertical deflection of the beam as a function of the vertical position of the aperture. Each collimator in the experiment was designed to present a relatively large transverse impedance and to minimize the impedance from other contributions such as resistivity. In addition, the collimator parameters were chosen to provide some insight into the scaling of the transverse geometric wakefield as a function of the collimator’s geometry. A description of the experimental apparatus and the aperture design, the method of data collection and analysis, and a comparison to theoretical and numerical predictions are presented.