An interpretation of magnetron problem

The electron motions inside a magnetron can be understood by dividing them into three stages. At low voltage the turbulent nature of electron motion may produce high electron temperature which, in turn, causes both violation of Hull's cutoff condition and back bombardment of the cathode. As the voltage reaches such a value that the average back bombarding energy makes the coefficient of secondary emission greater than one, a process of cyclic multiplication at the cathode surface may happen, and it has the property to transform the disorder motion into space-charge oscillation which has been repeatedly observed in smooth-bore magnetrons. When the predominating mode of the space-charge oscillation synchronizes with the π mode frequency of the cavity resonators, the magnetron oscillates.