Performance and parameters of a novel talbot effect confocal resonator for MM-wave FEL

The design, operating principles, and results of characterization for a novel resonator are outlined. Measurements were conducted prior to insertion into the Wiggler cavity for future testing under lasing. The W-band (75-110 GHz) resonator consists of two Talbot splitters and two confocal parabolic mirrors for decoupling the electron beam from the RF radiation, a corrugated waveguide, and an adjustable three grid reflector system. Two degrees of freedom have been built into the grid system, firstly, the central grid can be rotated via remote control to alter the out-coupling coefficient, and secondly, also using a motor it is possible to remotely move the grid system back and forth altering the length of the resonator allowing continuous tuning of the longitudinal mode resonant frequencies (spaced about 100MHz apart). The radiation pattern of the resonator mode is nearly a Gaussian. The input reflectance and Q factors where measured by matching the S parameters of the device (measured with a Scalar Netwok Analyser) to the theoretical Fabry-Perot resonator reflection and transmission curves. Based on this estimate the roundtrip losses of the tunable resonator are 37%.