  The effects of electron clouds on positively-charged beams have been an
active area of research in recent years at particle accelerators around the
world. Transverse beam-size blow-up due to electron clouds has been observed in
some machines, and is considered to be a major limiting factor in the
development of higher-current, higher-luminosity electron-positron colliders.
The leading proposed mechanism for beam blow-up is the excitation of a fast
head-tail instability due to short-range wakes within the electron cloud. We
present here observations of betatron oscillation sidebands in bunch-by-bunch
spectra that may provide direct evidence of such head-tail motion in a positron
beam.
