  We expand on the material that was published in the previous Proceedings of
the CERN Accelerator School on Plasma Wakefield Acceleration. The material
focused on Plasma Wakefield Acceleration in the short, narrow bunch regime.
After a brief introduction, we describe Plasma Wakefield Acceleration driven by
a bunch train. We then attempt to give simple and intuitive descriptions of
bunch self-modulation, occurring when the bunch is long, and of current
filamentation, occurring when the bunch is wide. Self-modulation is a means to
use existing bunches carrying large amounts of energy to drive plasma
wakefields. Current filamentation instability imposes a limitation on how wide
a particle bunch can be before transverse break-up may disrupt the wakefields
generation and the acceleration process. As in the previous material, we show
sample experimental results that demonstrate that much of the physics at play
has been observed experimentally.
