Emittance dilution and beam break up in the JLC/NLC

The passage of a multi-bunch beam through an X-band accelerator with cells and structures misaligned gives rise to wakefields which disrupt succeeding bunches. As a consequence, there is growth in the overall emittance of the final colliding bunches. Here we study this process in the context of a JLC-NLC which employs the recently developed RDDS (Rounded Damped Detuned Structures) accelerators with 120 s fill time and drives a 90 bunch, 250 ns bunch train. We investigate cumulative beam break-up (BBU) under various parameter regimes appropriate to the structure which is in the process of being fabricated by KEK and SLAC. We compute the resultant emittance growth by means of analytical expressions and find good agreement with the results we obtain from a particle tracking code. We set misalignment tolerances by specifying allowable emittance growth.