Strong wakefields generated by a train of femtosecond bunches in a planar dielectric microstructure

A tall, dielectric-lined rectangular wakefield microstructure is analyzed as a possible element of an advanced linear wakefield accelerator. This accelerator would be driven by a train of fs electron microbunches that would be chopped out of a longer bunch using a powerful CO₂ laser and then formed into a train of rectangular-profile bunches using a quadrupole. The bunches set up a periodic wakefield in the microstructure that can be built up to 400–600  MV/m, for example, using a train of ten 3-fs 1-pC bunches. Two major issues are examined. First, interference is studied using the particle-in-cell code KARAT between transition radiation and Cerenkov wakefield radiation, both set up by the passage of a charge bunch through a dielectric structure of finite length. Of significance is the difference in propagation speeds of transition radiation and Cerenkov radiation (which travels almost at the vacuum light speed c) and the magnitude of the respective fields. Second, stability is examined for drive and accelerated bunches using computations of test particle orbits in the longitudinal and transverse wakefields excited by the drive bunches. It is found that nearly all test electrons in the drive bunches are confined within the structure for a travel distance of 30 cm or more, while test electrons located in an accelerated bunch can have stable motion over greater than 30 cm without passing through the structure walls.