  Wakefield excitation by structured electron bunches in hollow gaps between
plasma wedges is studied using three-dimensional particle-in-cell simulations.
The main part of the electron bunch has a triangular current distribution in
the longitudinal direction with a smooth head and short tail. These bunches
propagate stably in the hollow gap while being attached to cusps of the plasma
wedges. The excited wakefield profile may have a very high transformer ratio
and allows to accelerate witness bunches to energies much higher than that of
the driver bunch. Unlike round hollow channels, where asymmetric wakefields are
difficult to avoid, no deleterious transverse beam break-up (BBU) is observed
in the gap between cusp-shaped plasma layers.
