Production of Terahertz Seed Radiation for FEL/IFEL Microbunchers for Second Generation Plasma Beatwave Experiments at Neptune

To achieve phase locked injection of short electron bunches in a plasma beatwave accelerator, the Neptune Laboratory will utilize microbunching in an FEL or IFEL system. These systems require terahertz (THz) seed radiation on the order of 10 kW for the FEL and 10 MW for the IFEL bunchers. We report results of experiments on THz generation using nonlinear frequency mixing of CO2laser lines in GaAs. A two-wavelength laser beam was split and sent onto a 2.5 cm long GaAs crystal cut for noncollinear phase matching. Low power measurements achieved ∼ 1 W of 340 μm radiation using 200 ns CO2pump pulses with wavelengths 10.3μm and 10.6μm. We also demonstrated tunability of difference frequency radiation, producing 240μm by mixing two different CO2laser lines. By going to shorter laser pulses and higher intensities, we were able to increase the conversion efficiency while decreasing the surface damage threshold. Using 200ps pulses we produced ∼ 2 MW of 340 μm radiation. Future studies in this area will focus on developing large diameter Quasi-Phase matched structures for production of high power THz radiation using collinear two frequency radiation.