Spectral Simulation of Mode Competition in a Magnetron with an Essential Influence of Space Charge

Abstract

The article investigates the excitation and competition of different types of oscillations (modes) in a resonant system of a typical 2450 MHz magnetron of medium power (1 kW), which are usually used in microwave ovens for household and corporate purposes. A feature of such magnetrons is the significant influence of space charge forces on the process of spoke formation during the mutual transformation of modes while the “hopping” of modes. Inevitably, in such cases, the expansion of the spectrum of the output signal can, under several conditions (primarily, inadequate quality of the high-frequency shielding system), significantly worsen the electromagnetic environment around the microwave device, which can lead to disruption of the operation of Wi-Fi communication systems in adjacent frequency ranges.

The research is carried out by the method “Particle in Cell” using the Magnol Problem of the TULIPgm application package. A two-dimensional approximation and a second-level spectral model (the method of complex amplitudes) are used. The spectra of the output signal and the induced current of the probe, which is in the middle of the interaction space, are analyzed. During the research, it was possible to simulate the transition process between the working (N/2) mode of oscillations and the high-voltage neighboring (N/2–1) mode. Further transition to the (N/2–2) mode does not occur, instead the destruction of the spoke grouping takes place. During the transition process, a significant (although relatively short-term) broadening of the spectra is observed. The spectrum of the output signal when operating on the N/2–1 mode is analyzed in detail, in particular, the preservation of the component with the frequency of the operating (N/2) mode of relatively large amplitude is established. The essential effect of the space charge in relatively low-power magnetrons leads to significant nonlinearity of the electron cloud, which sharply increases the amplitudes of the intermodulation components during the transition processes and creates the phenomenon of anode voltage hysteresis when switching from one mode of oscillation to another and vice versa. Based on the research performed, design solutions can be proposed for further optimization of the operating modes of the most common magnetron generators.