Magnetrons typically operate in the π mode, where the microwave electric fields at adjacent resonant cavity openings are exactly 180 ° out of phase, meaning that the direction of the microwave electric fields is exactly opposite.



Although this microwave field is a standing wave field, in the case of π mode, it is equivalent to two identical microwave fields moving in opposite directions on a circle, and the phase velocity values of the two fields are equal. Electrons emitted from the cathode undergo cycloid motion under the action of an orthogonal electromagnetic field. By adjusting the DC voltage and constant magnetic field, the average drift velocity of electrons in the circumferential direction v=E/B is exactly equal to the phase velocity v of a microwave field moving in its direction (where E is the average DC electric field generated by the DC voltage in the interaction space, and B is the axial constant magnetic induction intensity), and electrons can move synchronously with the microwave field. During synchronous motion, the electrons in the microwave deceleration field gradually transfer their DC potential energy to the microwave field and approach the anode, ultimately being collected by the anode. These electrons transfer energy to the microwave field, which is beneficial for establishing stable microwave oscillations in the magnetron, hence they are called favorable electrons. The part of electrons in the microwave acceleration field obtains energy from the microwave field and moves towards the cathode, finally hitting the cathode. These electrons are called unfavorable electrons.



When unfavorable electrons return to the cathode, a large number of secondary electrons are ejected, resulting in an increase in the number of interacting space electrons. The maximum deceleration zone is the center of electron clustering. The electrons on both sides of it are subjected to a force that moves towards the cluster center. The maximum acceleration field region is the center of electron scattering, where nearby electrons are subjected to forces that deviate from the center and move towards the left and right sides, transforming into favorable electrons. In this way, during the establishment of oscillation, the number of unfavorable electrons decreases and the number of favorable electrons increases, gradually concentrating towards the cluster center and forming a spoke shaped electron cloud in the interaction space. The phenomenon of electrons in different phases automatically clustering into a spoke shaped electron cloud in the interaction space is called automatic phase focusing. The microwave field in the interaction space decays exponentially as it moves away from the anode surface.



Therefore, the microwave field on the cathode surface is extremely weak, and the clustering effect on electrons is minimal. Near the cathode, no obvious electron spokes are formed, but almost uniformly distributed electron hubs are formed. Magnetrons have the majority of favorable electrons in the interaction space, and they are all moving towards the anode. The favorable electrons have a long time to spin, and they can fully convert DC potential energy into microwave energy; There are relatively few electrons that bounce back from the cathode, and they hit the cathode shortly after being emitted, thus absorbing less energy from the microwave field. In this way, the total effect of the interaction between all electrons in the interaction space and the microwave field is that the electrons transfer their DC potential energy to the microwave field, establishing stable microwave oscillations in the magnetron.