Impact of narrow gap undulators on the Advanced Light Source

In low emittance synchrotron light sources the main source of beam loss is intrabeam or Touschek scattering. In most cases the Touschek scattered particles get lost on the vacuum chamber with the narrowest vertical gap. The particles reach the vertical chamber from either diffusion or coupling of the particle motion from the horizontal to the vertical plane. The beam lifetime can be very sensitive to the size of the vertical chamber that can limit the dynamic momentum aperture. The reduction in lifetime limits the minimum size of the chamber that can be installed in the ring. In this paper we examine the effect of the size of the vertical apertures on the beam lifetime at the ALS under various different machine conditions. We show that there are conditions where one can make the beam lifetime much less sensitive to the size of the vertical aperture.