Electron Autoacceleration and Efficient Microwave Generation in a Radial Three-Cavity Transit-Time Oscillator With Two Output Ports

High power microwave (HPM) sources without guiding magnetic field have the advantages of compactness and miniaturization. However, the efficiency is generally limited due to the poor beam quality and low beam transmission rate. Here, we demonstrate a scheme that actually makes use of the large oscillation of electrons in the absence of a magnetic field. In a novel radial three-cavity transit-time oscillator (TTO) with two output ports, about half of the diode current is deposited on the walls of the first two cavities. On the one hand, the truncated electrons are with lower energy, which has delivered some of their kinetic energy to the standing-wave field in the first two cavities. Other electrons then gain energy from the standing-wave field and are accelerated to a much higher beam voltage than the diode voltage. On the other hand, after part electrons are truncated by the first two cavities, the remained electrons are characterized with smaller energy spread, higher current modulation coefficient, which results in a very efficient energy exchange between the beam current and the electromagnetic field in the third cavity with two extraction gaps. Particle-in-cell (PIC) simulation shows that when the diode voltage is 496 kV, beam current is 43.4 kA, the total output microwave powers are 11.4 GW, corresponding to an unprecedented beam-wave conversion efficiency of 53%.