11-1 



THE MAGNETRON 



581 



Ideal Theoretical 



Experimental 



Higher Voltage 



Fig. 11-2 Electron Trajectories in a 

 Static, Nonoscillating Magnetron. 



Fig. 11-3 Voltage-Current Relationship 

 in a Nonoscillating Magnetron. 



cycloidal path and returns to the cathode, as shown in Fig. 11-2. As the 

 voltage is increased, the maximum distance from the cathode reached by 

 the electron also increases. Finally, as a critical voltage is reached, the 

 electron will strike the anode. The critical voltage is given by 



['-©■]■ 



(11-1) 



where Vc is the voltage, B the magnetic field, e jm is the charge to mass ratio 

 of the electron, and Tc and Va the radii of the cathode and anode respectively. 



If this ideal theoretical behavior were actually followed, the voltage 

 current relationship would be as shown in Fig. 11-3. Experimentally, the 

 abrupt transition at the critical cutoff voltage is not observed, although a 

 rapid increase of current is observed, as shown. The reasons for this 

 departure from theory are not well understood, although it has been 

 postulated that electronic interaction of some nature takes place in the 

 whirling cloud of electrons with crossing trajectories that surrounds the 

 cathode. The possibility of an electronic interaction in the whirling cloud 

 of space charge is strengthened by another experimentally observed fact. 

 Some of the electrons that return to the cathode strike it with considerable 

 velocity, again in contradiction to the simple theory. These electrons 

 release secondary electrons which contribute to the electron emission of 

 the cathode. Cathode emissivity depends then upon both primary and 

 secondary emission properties. 



In normal operation of a magnetron, the heater voltage must be turned 

 on initially to heat the cathode to a temperature sufficient for electron 



