Microwave quadrupole structures for the CERN Linear Collider

Considerable transverse focusing power can be obtained from the high-gradient RF accelerating structure of a linear collider by making either the aperture or the main cell geometry asymmetric. Such structures, acting as microwave quadrupoles, can be used to stabilize transverse wakefields by creating a spread in the transverse oscillation frequencies of the particles within a bunch. The focusing properties of both slotted-iris structures with circular cells and circular-aperture structures with asymmetric cells have been analyzed using the MAFIA computer code and the results compared with the theoretically determined limiting value obtained for an infinitely thin slit. A suitable geometry for CLIC (CERN Linear Collider) 30-GHz structures has been established and a design based on the machine-and-braze technique is proposed.<>