Development of a THz seed source for FEL microbunching experiment at the neptune laboratory

Seeded FEL/IFEL techniques can be used for modulation of a relativistic electron beam longitudinally on the radiation wavelength. However, in the 1-10 THz range, which is of particular importance for the matched injection of prebunched electrons into a laser-driven plasma accelerating structure, a suitable radiation source is not available. At the UCLA Neptune Laboratory we have built and fully characterized a radiation source tunable in the range of 1-3 THz. The THz pulse is produced by mixing two CO₂ laser lines in a noncollinear phase-matched GaAs crystal at room temperature. The crystal is pumped by 200 ns pulses of a dual beam TEA CO₂ laser running at 1 Hz. A grating placed in each lasing section allowed to cover the spectral range for the difference frequency from 0.5-4.5 THz with a step of 30-40 GHz. The achieved narrow bandwidth (Deltav/v ~10^-5) and the output power of 2 kW are sufficient for seeding a single-pass waveguide FEL amplifier-prebuncher. These pulses were used to measure the coupling efficiency and the attenuation for the different types of THz waveguides and the results will be reported.