Beam instabilities related to different focusing schemes in TESLA

The preservation of beam emittances during the transport through the main linac is essential to achieve maximum luminosity. An increase of the beam area in transverse phase space occurs when beam trajectories are displaced from the accelerator axis: chromatic effects due to the finite energy spread are generated by the focusing magnets while beam instabilities due to the excitation of wakefields are induced by the accelerating structures. The major sources of emittance growth of a beam accelerated in the TESLA main linac are studied for different focusing schemes, including the constant /spl beta/ and the energy dependent /spl beta/ lattices. Since the accelerating structures have a low RF frequency and a large iris aperture, very weak or even no BNS damping is necessary to cure the single bunch instability from coherent betatron oscillations. In addition, different orbit correction techniques have been tested and the resulting emittance dilutions are compared. However, the choice of the steering method will be determined mainly by the effects of the wakefields, both short-range and long-range, induced by the randomly displaced cavities, which are allowed to have a large scatter in their misalignments.<>