  We report preliminary studies of a three-channel, rectangular, high gradient
dielectric wakefield accelerator element, which, unlike the collinear
cylindrical dielectric wakefield device, does not suffer from low transformer
ratio and may offer relief from the beam breakup instability. When configured
as a resonator (DWR), it can be driven by a series of modest-charge drive
bunches. This rectangular 'mode-locked' resonator consists of three channels
lined with low-loss dielectric slabs: the central wider channel is the drive
bunch channel, whereas two adjacent narrow channels are used to accelerate
electron and/or positron bunches; this provides a favorable transformer ratio.
At the moment when, after reflecting from the resonator exit, the wakefield
returns to the resonator entrance, the next bunch is injected into the
resonator entrance. The length of the resonator is also chosen to be a multiple
of half the desired wakefield wavelength. The rectangular geometry permits
superposition of harmonics of wakefields in the resonator. The short length of
the device and its planar configuration should allow management of beam
breakup, and a lengthy drive bunch train can be dynamically stabilized by using
a series of DWR units rotated about the $z$-axis in $90^{\circ}$ increments.
