Wigglers, beam emittance and energy spread

Wigglers are used in storage rings to increase the beam emittance for maximum luminosity. If, however, a storage ring is used as a synchrotron light source a beam emittance as small as possible is desired. Also in special cases in high energy physics research a large luminosity achieved with wigglers may not be the optimum running configuration. Together with an increase of the beam emittance also the energy spread of the beam is increased. This increase reduces the eventrate for narrow resonances like the /psi/ or /Tau/. In this note we will investigate the conditions for a reduction in beam emittance and/or of the energy spread in the beam.