Wakefields of short bunches in the canal covered with thin dielectric layer

In new generation of accelerators shorter and shorter bunches are considered. In addition to the usual wakefields, excited in corrugated or resistive vacuum chambers, other harmful wakes can be excited in smooth beam tubes, covered by thin dielectric layer. Analytical approximation and results of computer simulations are presented for various bunchlengths and dielectric thickness. It is shown that a 50 μm long bunch, travelling through a tube with ceramic coating of only 0.1 μm gives rise to large amplitude wakefields, comparable to usual wakes in accelerating cavities. This effect can be dramatic for long transport-lines. Dielectric coatings in the tubes can be of different types. A few examples, like oxide layer in aluminum tubes, are finally discussed. Derived estimations are also given for corrugated metallic surfaces, which can have similar behaviour to dielectric coated tubes