  Interaction between the bunches in the NLC main linac via long range
wakefields can cause a beam breakup instability. Although the magnitude of the
long range wakefields for an ideal NLC structure is below the instability
threshold, the wake increases when structure manufacturing errors are taken
into account. In case of large errors, the developing instability can result in
the projected emittance dilution of the bunch train. To suppress the
instability, we propose to introduce an energy spread between the bunches,
similar to the BNS energy spread for damping of the beam breakup within a
single bunch. Based on simple estimates, we show that the energy spread of
order of 1-2% should be enough for suppression of the instability. The results
of computer simulations with the simulation code LIAR confirm theoretical
estimates and indicate that both the tolerances for structure misalignments and
the incoming beam jitter can be considerably loosened by introducing the energy
spread within the beam.
