The UCSB MM-FEL, FIR-FEL, and 30 µm FEL all employ overmoded, parallel plate, waveguide resonators, terminated with cylindrical mirrors.
illustration of resonator
MM-FEL5.93 m1.9 cm12.7 cm2.97 m3.60 m
FIR-FEL5.36 m1.05 cm7.6 cm3.0 m3.0 m
30µm-FEL5.27 m5.0 mm5.5 cm2.74 m2.68 m

These offer unique advantages for FELs that operate at long wavelength:

measured loss fit to an exponential The FIR-FEL decay time, measured with a Schottky detector and fast digitizer, is fitted to an exponential to measure loss.
loss budget table The table shows estimated contributions to this loss, where rc is coupling coefficient, lc is coupling loss (backscattering to higher order modes), lwg is resistive waveguide wall loss, ld is mirror edge diffraction loss, lm is mirror reflection loss, and lt is total loss.