584 



THE GENERATION OF MICROWAVE POWER 



Mechanism of Oscillation. The mechanisms that lead to oscillation 

 in a magnetron may be understood by considering first what happens when 

 a voltage is applied between cathode and anode in the magnetron. Although 

 no current is drawn initially to the anode, a whirling cloud of electrons is 

 formed about the cathode that expands in radius as the voltage is increased. 

 Neglecting the electronic interaction effects, the outer radius of the cloud 

 will reach the anode when the cutoff voltage is reached. For voltages below 

 the cutoff voltage, the electrons near the outer edge of the cloud have 

 considerable angular velocity as they sweep by the slots in the anode. 



If an RF voltage exists across the slots such that the electron picks up 

 energy from the field, its path will be distorted in a manner such that the 

 electron will be returned to the cathode. It will strike the cathode and be 

 removed from the cloud, although it may be expected possibly to release 

 some new secondary electrons into the electron cloud. On the other hand, 

 an electron which delivers energy into the RF field and is thereby slowed 

 down will not be returned to the cathode, but will continue to travel in 

 the cloud. There therefore exists a sorting mechanism, whereby the 

 electrons that extract energy from the RF field are removed from the 

 electron cloud, and those that deliver energy to the field remain in the cloud. 

 A condition of synchronism between the rotating electrons and the 

 electromagnetic field must also exist for efficient operation. For the normal 

 TT mode of operation, at any given instant of time the electric field across 

 one of the slots is exactly 180° out of phase with the field across the slots 

 adjacent to it on each side. If the conditions of synchronism are met, the 

 angular velocity of the electrons will be such that in the time it takes an 

 electron to travel from one slot to the next, one half-cycle will have elapsed. 

 The rotating electron will therefore encounter RF fields of identical phase 

 as it passes by successive slots. Equivalent conditions of synchronism exist 

 for the other possible modes of oscillation. 



When the conditions of synchro- 

 nism are met and the voltage is 

 below the cutoff voltage, the sorting 

 mechanism will begin to operate and 

 there will be a net transfer of energy 

 from the electron cloud to the RF 

 field, causing oscillations to build up. 

 The electron cloud will be distorted 

 into the form of a spoked wheel, with 

 N 12 spokes if there are A'' cavities, as 

 shown in Fig. 11-8. These spokes 

 sweep by the slots in the proper 

 Fig. 11-8 Rotating Space-Charge Cloud phase for the electrons to deliver 

 in an Oscillating Eight-Resonator Mag- ^ ^^ ^j^^ ^VF field, and the 



netron. 



