  We have directly measured THz wakefields induced by a subpicosecond, intense
relativistic electron bunch in a diamond loaded accelerating structure via the
wakefield acceleration method. We present here the beam test results from the
first diamond based structure. Diamond has been chosen for its high breakdown
threshold and unique thermoconductive properties. Fields produced by a leading
(drive) beam were used to accelerate a trailing (witness) electron bunch which
followed the drive bunch at a variable distance. The energy gain of a witness
bunch as a function of its separation from the drive bunch describes the time
structure of the generated wakefield.
