The possibility of introducing additional focusing caused by the circular irises in iris loaded accelerator sections

To reach high luminosities in future linear colliders short range wakes have to be controlled in the range of X-band frequencies or higher. Rectangular irises can be used to introduce strong focusing quadrupole-like RF-fields. Even circular irises in iris-loaded accelerator structures have the capability of focusing if the particle velocity differs from phase velocity. Theoretical investigations concerning the focusing strength to be expected are presented. Their applicability for linear colliders is discussed.<>