Experimental study of a large diameter backward wave oscillator without guiding magnetic field

A large diameter backward wave oscillator (BWO) without guiding magnetic field is demonstrated experimentally. The beam propagation is achieved by filling slow wave structure (SWS) with helium or nitrogen gas. The microwave power peaks at nearly 120 mtorr for helium gas and at nearly 8 mtorr for nitrogen gas. Two distinct radiation modes from the BWO are observed and are controlled by the applied beam voltage. One is dominated by axisymmetric TM modes and the other is dominated by non-axisymmetric TE modes. The maximum output power is of the order of 1 kW, which corresponds to electronic efficiency less than 0.1 %. A self-consistent field theory naturally predicts oscillations of non-axisymmetric TE as well as axisymmetric TM. Radiation modes from the BWO without magnetic field can be explained by interactions between normal electromagnetic modes of SWS and degenerate slow space charge beam mode at zero-magnetic field.