Powerful FEM-oscillators with advanced Bragg resonators operating in a single mode regime from Ka- to W-band

The paper describes recent results of theoretical and experimental studies of the FEM-oscillator driven by linac LIU-3000 0.8 MeV / 200 A / 200 ns. At the present stage efficient operation of the JINR-IAP FEM has been demonstrated at 30 GHz / 15 MW and 60 GHz / 5 MW powers. Project of multimegawatt power 90 GHz FEM is in progress currently. The key components of the FEM are short period tapered wiggler, which is responsible for high-quality helical electron beam formation, and advanced Bragg resonator based on coupling of propagating and quasi-cutoff waves that improves selectivity over transverse coordinates and allows advance of FEM into short wavelengths.