Project of a large orbit FEM-oscillator operated at the second harmonic

To operate at harmonics of the bounce-frequency in the JINR-IAP FEM-oscillator driven by the LIU-3000 linac (JINR, Dubna) we propose using a large electron orbit regime. In this regime a thin axial beam moving in a helical wiggler with an axially guided magnetic field excites at the nth bounce harmonic only the waveguide mode with the same azimuthal index. For effective FEM operation in this regime, the radius of electron orbit should be comparable with the transverse inhomogeneity of the resonant wave. A 75 GHz FEM project for operation at the second harmonic of the bounce-frequency is presented.