Transverse wake fields in the CLIC transfer structure

The transfer structure for generation of microwave power from the bunched drive beam presently consists of a smooth beam tube and a periodically loaded wave guide running in parallel and coupled to it by a slot. The bunches are in synchronous interaction with a forward 2π/3-mode. Both the coupling from the beam to the output wave guide and the wave propagation down the tube have been measured on a scaled model at 8.6 GHz with the wire method, giving the longitudinal beam impedance and wake field. Transverse wake fields can be calculated from the longitudinal ones measured with the wire close to a shorting plate introduced in the centre of the structure. Quantitative estimates are then deduced for the real-scale transfer structure at 30 GHz and compared with results obtained from the three-dimensional MAFIA package, both being in good agreement. While the longitudinal wake fields from leading bunches are in phase with the following bunches, the transverse wake fields have a 90° phase offset. The wake field functions resulting from these studies have been inserted into the tracking program DTRACK and the transverse beam blow-up obtained seems to indicate that the effects of such wake fields remain tolerable