Seeded FEL Amplifier-Buncher in the 0.5-9 THz for Advanced Accelerators

Longitudinal modulation of a relativistic electron beam in the THz range is important for advanced laser- or beam-driven plasma accelerators operating in the 1016-1018 cm-3 plasma density range. We describe a single-pass FEL amplifier-buncher which is under construction at the UCLA Neptune laboratory. Microbunching on the 0.5-3 THz frequency scale is achieved during the process of a resonant FEL interaction between an electron beam and a THz seed pulse. A narrow-band, low-power THz seed source based on the frequency mixing of CO2 laser lines in a GaAs nonlinear crystal is built and fully characterized. The THz radiation pulse generated by this source will be guided through a hollow waveguide inside the planar FEL undulator driven by a regular photoinjector. By using a time-dependent FEL code GENESIS 1.3, we optimized the undulator parameters and analyzed the dynamics of the modulated electron beam. At present, the THz FEL microbuncher is being built and we update the status of the project.