Numerical study of interference between transition radiation and Cerenkov wake field radiation in a planar dielectric structure

The PIC code KARAT is used to study the interference between transition radiation and Cerenkov wake field radiation, set up by the passage of a bunch of charge through a dielectric structure of finite length. An example studied is a tall, dielectric-lined rectangular wake field microstructure, recently proposed as a stageable element of an advanced linear accelerator, which would use a train of femtosecond duration bunches. These bunches would be chopped out of a longer bunch using a powerful CO₂ laser and formed into a rectangular-profile bunch by a quadrupole. The bunches set up a periodic wake field which can be built up to as much as 600 MV/m using ten 3-fs bunches each containing a charge of 1-pC. Of interest is the difference in relative propagation speeds of the transition radiation and the Cerenkov radiation (which advances almost at c), and the relative magnitudes of the fields.