The THz frequency regime holds the possibility of a new frontier in advanced accelerator research. In bridging the gap between optical and RF technology, THz-based accelerating structures can retain advantages from both, achieving high field gradients while maintaining large temporal acceptance relative to beam size. These features make THz radiation an exciting tool for beam manipulation, but, thus far, exploration of its application to accelerator physics has been limited by the low power of today's THz source technology, typically limited to a uJ-level pulse energy, often in a near single cycle waveform.

In this dissertation, we present a guiding technique for phase and group velocity matched interaction between a near single cycle THz pulse and an electron beam copropagating in a magnetic undulator. This "zero-slippage" scheme results in efficient energy exchange, necessary for utilizing a low power THz source, and in extended interaction, necessary for harnessing short and intense THz pulses, along with unique features like broadband coupling and tunable resonance that stem from the waveguide-induced dispersion. We explore the applications of "zero-slippage" coupling in an inverse free electron laser (IFEL), for THz-driven acceleration and beam manipulation, and a free electron laser (FEL), for broadband THz amplification or a stand-alone THz source based on spontaneous superradiance.

To model this novel interaction regime, we developed a 1-dimensional FEL simulation tool that tracks the THz pulse undergoing dispersion in a waveguide, along with the longitudinal phase space dynamics of the beam. The experimental work presented in this dissertation includes a THz IFEL and FEL experiment, both conducted on the PEGASUS beamline at UCLA using a 30 cm, planar, permanent magnet undulator and 1 uJ-level laser-based THz source. The THz IFEL experiment produced a record 150 keV energy modulation of the relativistic beam, verified the tunable resonance of the guided IFEL interaction, and provided longitudinal phase space measurements demonstrating potential applications of the technique, including bunch compression by a factor of two.

Measurements from the THz FEL experiment show evidence of both stimulated amplification/absorption of the THz seed pulse, and spontaneous superradiant emission, due to the short bunch length relative to THz wavelength after bunch compression driven by the PEGASUS linac. Extrapolating to a 100-200 pC beam in a tapered undulator, simulations predict corresponding THz outputs exceeding 100 uJ with up to 30 % beam energy extraction, from broadband amplification with a long beam or spontaneous superradiance from a short beam, inviting the development of exciting and competitive new THz sources using the "zero-slippage" FEL technology.