  We describe an IFEL interaction driven by a guided broadband THz source to
compress a relativistic electron bunch and synchronize it with an external
laser pulse. A high field near single-cycle THz pulse, generated via optical
rectification from the external laser source, is group velocity-matched to the
electron bunch inside a waveguide, allowing for a sustained interaction in a
magnetic undulator. We present measurements of the THz waveform before and
after a curved parallel plate waveguide with varying aperture size and estimate
the reduced group velocity. For a proof-of-concept experiment at the UCLA
PEGASUS laboratory, a 6 MeV, 100 fs electron bunch with an initial 0.03 %
energy spread can be readily produced. Given these parameters and a projected
THz peak field of 10 MV/m, our simulation model predicts a phase space rotation
of the bunch distribution that compresses the electron bunch by nearly an order
of magnitude and reduces any initial timing jitter within the phase acceptance
window. We also discuss the application of this guided-THz IFEL experimental
set-up towards a THz streaking diagnostic with the potential for femtosecond
scale temporal resolution.
