The cyclotron-resonance maser (CRM) interaction is demonstrated in a periodic-waveguide which consists of a 2D array of metal posts. The CRM employs a low-energy electron beam (approximately 5 keV, 0.1 A) in an axial magnetic field (approximately 3 kG). Microwave output signals are observed in microwave frequencies around 7 GHz. The conversion efficiency is approximately 10%.
"Cyclotron maser experiment in two-dimensional periodic array", Proc. SPIE 2843, Intense Microwave Pulses IV, (28 October 1996); doi: 10.1117/12.255406; https://doi.org/10.1117/12.255406