A Ka-band coaxial transit time oscillator with a focusing cathode

A low guiding magnetic field facilitates the compactness of high power microwave sources, thus making it more practical. In order to reduce the guiding magnetic field, a focusing cathode is introduced in a Ka-band coaxial transit time oscillator in this paper. Based on the focusing cathode with a large impedance (>100 Ω) coaxial structure, a particle-in-cell simulation is conducted. When the diode voltage is 480 kV and the beam current is 3.62 kA, a microwave with an average power of 800 MW at 31.667 GHz is obtained under a guiding magnetic field of 0.5 T, the corresponding conversion efficiency is 46%, and the saturation time is 25 ns. Meanwhile, the maximum EZ field in the diode region is 350 kV/cm; thus, electron emission beyond the cathode region is avoided. When the guiding magnetic field is 0.5 T and the voltage varies in the range of 430–510 kV, the conversion efficiency is greater than 40%. In addition, when the voltage stays at 480 kV and the guiding magnetic field is in the range of 0.45–1.0 T, the conversion efficiency is also greater than 40%.