  One approach to future high energy particle accelerators is based on the
wakefield principle: a leading high-charge drive bunch is used to excite fields
in an accelerating structure or plasma that in turn accelerates a trailing
low-charge witness bunch. The transformer ratio R is defined as the ratio of
the maximum energy gain of the witness bunch to the maximum energy loss of the
drive bunch. In general, for configurations in which the two beams traverse the
accelerator along the same trajectory (collinear wakefield acceleration). A
number of techniques have been proposed to overcome the transformer ratio
limitation. We report here the first experimental study of the ramped bunch
train (RBT) technique in which a dielectric loaded waveguide was used as the
accelerating structure. A single drive bunch was replaced by two bunches with
charge ratio of 1:2.5 and a separation of 10.5 wavelengths of the fundamental
mode. An average measured transformer ratio enhancement by a factor of 1.31
over the single drive bunch case was obtained in this experiment.
