  We consider the quasi-particle excitations of a trapped dipolar Bose-Einstein
condensate. By mapping these excitations onto radial and angular momentum we
show that the roton modes are clearly revealed as discrete fingers in parameter
space, whereas the other modes form a smooth surface. We examine the properties
of the roton modes and characterize how they change with the dipole interaction
strength. We demonstrate how the application of a perturbing potential can be
used to engineer angular rotons, i.e. allowing us to controllably select modes
of non-zero angular momentum to become the lowest energy rotons.
