  The RadiaBeam/SLAC dechirper, a structure consisting of pairs of flat,
metallic, corrugated plates, %a corrugated structure in flat geometry, has been
installed just upstream of the undulators in the Linac Coherent Light Source
(LCLS). As a dechirper, with the beam passing between the plates on axis,
longitudinal wakefields are induced that can remove unwanted energy chirp in
the beam. However, with the beam passing off axis, strong transverse wakes are
also induced. This mode of operation has already been used for the production
of intense, multi-color photon beams using the Fresh-Slice technique, and is
being used to develop a diagnostic for attosecond bunch length measurements.
Here we measure, as function of offset, the strength of the transverse
wakefields that are excited between the two plates, and also for the case of
the beam passing near to a single plate. We compare with analytical formulas
from the literature, and find good agreement. This report presents the first
systematic measurements of the transverse wake strength in a dechirper, one
that has been excited by a bunch with the short pulse duration and high energy
found in an X-ray free electron laser.
