Issues in multi-bunch emittance preservation in the NLC

In the linac of the SLAC NLC design 1 nC bunches are accelerated in trains 90 bunches long with an interbunch spacing of 42 cm. In this multi-bunch design one important problem that needs to be controlled is the multi-bunch beam break-up instability. One method of controlling this instability is by detuning the transverse modes of the accelerator cavities. This is accomplished by varying the cell dimensions as one proceeds down the structure in such a way that the transverse modes are detuned, while the fundamental, accelerating mode is left unchanged. In a properly designed, gaussian detuned structure the transverse wakefield excited by the first bunch in the train can be made to cancel sufficiently at the positions of bunches 2 to 25, after which it begins to grow again. To accommodate the long NLC bunch train it was suggested to build the linac out of 4 types of structures whose modes are interleaved so that the recoherence of the wake does not begin until after the last bunch has passed. More recently, another way of avoiding the recoherence of the wake is being studied: using only one type of structure and introducing weak damping through a manifold coupled by radial slots. In this paper, however, we will consider only the earlier idea. An important consideration in the NLC linac is the tolerance to accelerator structure misalignments. We present here an approximate analytic calculation that works well for the NLC parameters with 4 structure types.