MAGNETRON AS GENERATOR OF CENTIMETER WAVES 201 



and magnetic field increases the electronic efficiency. The dependence of 

 electronic efiiciency on B predicted by this simple picture is in accord with 

 the dependence predicted by more sophisticated theories. 



In all i)r()bability the decrease of electronic efficiency toward low and high 

 currents is, in part at least, the result of decrease in the phase focusing action. 

 This occurs as a result, on the one hand, of low RF field strength in the 

 proper mode when the current and RF oscillation are small, and, on the 

 other hand, as a result of space charge debunching when the current and 

 space charge become very large. In addition, at low currents the leakage to 

 the anode of electrons which are not effective in interaction with the RF 

 field, assumes a more dominant role, reducing the effective electronic effi- 

 ciency. These electrons are no doubt those emitted near or at the ends of 

 the cathode. 



Of importance to the motion of electrons near the ends of the interaction 

 space, and thus to the electron leakage, are the configurations of the DC 

 electric and magnetic fields there. These depend upon the geometry of the 

 cathode ends and surrounding walls and of the magnetic pole pieces. The 

 electrons are largely confined to the interaction space by the axial force, 

 directed toward the center of the interaction space, produced by the non- 

 uniformities in the electric and/or the magnetic fields. For uniform mag- 

 netic field, the desired focusing action on the electrons may be achieved by 

 disks at cathode potential which are mounted at each of the cathode and 

 extend beyond the cathode surface over the ends of the interaction space as 

 may be seen in Fig. 1. In other cases, distortion of the magnetic field in 

 the end spaces of the magnetron, in addition to cathode end disks, has been 

 used to produce the same efifect. 



Although the dependence of operation of a magnetron oscillator on load is 

 primarily a circuit problem, detailed discussion of which will be delayed until 

 the RF circuit has been discussed, there is one feature of the dependence on 

 load and circuit characteristics which may properly be discussed now. This 

 is the dependence of the electronic efficiency, r]e, on the circuit conductance, 

 G„ presented to the electrons. The plot in Fig. 19 is a typical example and 

 shows an optimum value of conductance, to each side of which nje decreases. 

 With decreasing Gs the RF voltage increases. Whereas initially the increase 

 of Vrf with decreasing G, increases the phase focusing properties, it results 

 eventually in such strong RF field that electrons are drawn more or less 

 directly to the anode where they arrive with considerable kinetic energy. 

 On the other hand, the decrease in Vrf with increasing G, eventually will 

 result in an RF field too weak to produce the necessary amount of phase 

 focusing. The value of G, presented to the electrons depends not only 

 upon the output circuit properties and load but also upon the parameters of 

 the resonator itself. The dependence of electronic efficiency upon G, is a 



