UCLA

Despite the rapid development of novel THz sources in the last few decades, free electron lasers remain attractive due to their unique advantages including frequency tunability and high peak power. While most THz-FEL facilities use optical cavities to build power over many passes, the ability to extract a significant fraction of the beam energy in a single pass paves the way towards high average power as the repetition rate of electron sources is increased.

In this dissertation, we study theoretically and experimentally a compact THz-FEL for improved single pass efficiency. At long wavelengths, FEL gain is limited by diffraction as well as slippage between the radiation and electron beam, requiring long bunch lengths for a sustained interaction. The introduction of a waveguide transversely confines the radiation and can be chosen to match the subluminal group velocity to the longitudinal electron beam velocity. This so-called zero-slippage operation extends the interaction length for short, high current beams allowing us to leverage developments in high brightness RF photoinjector sources. Strong seeding with a prebunched beam enables large decelerating gradients where resonance is maintained with strong undulator tapering, enhancing the extraction efficiency.

After beginning with the theory of waveguide FELs and zero-slippage resonance, we present our simulation code GPTFEL, a custom element built on top of the 3D particle tracking code General Particle Tracer. The code simulates free-space and waveguide FEL interactions by decomposing the electromagnetic fields into a basis of frequency and spatial modes, evolving the complex amplitudes according to energy conservation with the beam and enabling start-to-end beamline simulations within a single code. Waveguide dispersion is naturally included and for free-space interactions, a source dependent expansion is implemented to limit the required number of transverse modes.

Two Tessatron experiments were performed on the UCLA Pegasus beamline using a meter long, helical undulator designed for maximum FEL coupling. The undulator commissioning used both pulsed-wire and Hall probe measurements to minimize trajectory and phase errors. Due to the enclosed geometry and large beam trajectory amplitude, it was necessary to develop a 3D pulsed-wiring technique to align the wire and tune higher order field moments.

The first experiment demonstrated 10\% energy efficiency from a 200~pC electron beam at 165~GHz by seeding with the beam compressed to sub-wavelength scale. To enhance the spectral range, a second experiment utilized beamline upgrades including laser shaping on novel photocathodes and a compact permanent magnet chicane to prepare a prebunched, multipeak charge distribution at the undulator. By operating with a beam energy above zero-slippage resonance, the frequency was tunable over the experimental energies from 500 to 700~GHz. Future paths of investigation, including resonance with higher waveguide modes or a rectangular geometry, can improve charge transmission and are promising avenues to further expand the spectral reach and application of the source.