  Free electrons travelling in vacuum carry evanescent electric field, which
can only be coupled to free space light via interaction with a surrounding or
nearby medium: as Cherenkov radiation when travelling faster than the local
velocity of light or as "Smith-Purcell" (SP) radiation when passing over the
surface of a grating. The SP emission characteristics of simple gratings such
as a regular array of wires or slits are a well-understood phenomenon. Here we
show that for a compound grating made up of several closely-spaced slits
repeated for each period, the characteristic angular dispersion of
Smith-Purcell radiation can be selectively attenuated or enhanced. We analyze,
in particular, the change in intensity as slit elements are added to the
structure for two slit sizes and how this affects emission enhancement.
