Single-mode and multimode operation conditions in JINR-IAP millimeter-wave FEL-oscillator

The influence of the Q-factor of a two-mirror Bragg resonator on the efficiency and spectrum of radiation was studied both theoretically and experimentally for the JINR-IAP millimeter-wave free-electron laser (FEL)-oscillator driven by a linear induction accelerator (1 MeV/200 A/200 ns). In accordance with theoretical predictions, single-frequency operation in a low (optimal) Q-factor resonator and multifrequency operation in a high Q-factor resonator were observed. For an optimal resonator configuration at the frequency 31 GHz, single-mode generation with power of approximately 35 MW and efficiency of 26% was obtained. A significant drop in FEL efficiency was observed for multifrequency generation.