Variable height slot-outcoupling for the compact UH THz-FEL

The quest for tunable, compact coherent radiation sources in the THz region of the electro-magnetic spectrum is growing and stimulating further developments and research activities in this area. The THz-FEL Group of the University of Hawaii (UH) pursues the concept of minimizing size and cost of a recirculating-beam electrostatic accelerator THz-FEL. The system employs a low voltage (1.7 MeV) pelletron accelerator in conjunction with a short period (λu=8 mm) hybrid undulator and miniature beam line components. The continuous tunability offered by the system over a spectral range between 230 and 640 μm necessitates a broad-band outcoupling mechanism. Holes on the axis in resonator mirrors have been previously employed in long wavelength FELs as a means to couple out optical radiation from the cavity. The UH THz-FEL adopted a parallel plate waveguide resonator design introducing a height adjustable slot-aperture on one of the cylindrical metal mirrors. Based on the mode-matching method employed in treating waveguide discontinuities, we examine the characteristics of the chosen outcoupling mechanism and its influence on the intracavity optical fields.