Transverse effects of longitudinal wakefields at high dispersion.

摘要

In high energy linear colliders the transverse beam emittance has to be preserved in order to achieve small interaction spots. Beams with trajectory offsets in cavities excite transverse wakefields which kick the tail of the beam leading to an undesired emittance growth. Here we will concentrate on the longitudinal wakefield creating an energy deviation (Delta)E(sub W) within the beam. At high dispersion (eta) the beam will be spread out corresponding to its initial (and/or correlated) energy spread (Delta)E(sub 0) (or(Delta)E (sub rf)) and is therefore very sensitive to energy changes. The energy variation (Delta)E(sub W) will cause a transverse emittance blow up in the high dispersion regions. The effect can be estimated by comparing the betatron size (sigma)(sub x)=(radical)(beta)(epsilon)with displacement (Delta)(sub x)=(eta)E(sub W)/E. Some kicks and displacements will compensate each other along the beam line. Simulation results are presented showing how much is really compensated and the final emittance contribution of this effect for the SLC Ring-To-Linac transport line. To minimize it, any vacuum pipe irregularities, like bellows, diameter steps, collimators, etc. should be smoothed or avoided at higher dispersion areas.