MAGNETRON AS GENERATOR 01' CENTIMETER WAVES 177 



For the cylindrical case, the relation may be shown to be 



(8) 



-■■•-ff- -(;;)■]■ 



in terms of cathode and anode radii, r< and fa- 



2. Types of Magnetron Oscillators 



2.1 Definitions: The DC magnetron may be converted into an oscillator, 

 suitable for the generation of centimeter waves, by introducing RF fields into 

 the anode-cathode region. This may be done by applying between anode 



MAGNETIC FIELD, B 



Fig. 5.— Variation of current passed by a cylindrical DC magnetron at constant voltage, 

 plotted as a function of magnetic field. The orbits of electrons occurring at four different 

 magnetic fields are shown above the corresponding regions of the current characteristic. 



and cathode RF voltage from a resonant circuit, in which case the electrons 

 interact with the superposed radial RF field. Or, it may be done by split- 

 ting the magnetron anode into two or more segments between which the 

 RF voltage is applied. Then the electrons interact with the fringing RF 

 fields existing between the segments. The problem of understanding the 

 electronics of the multicavity magnetron oscillator is that of understanding 

 how an electron, subject to the constraints placed upon its motion by the 

 DC axial magnetic and DC radial electric fields, can move so as to interact 

 favorably with the RF field; how an electron interacting unfavorably is 

 rejected; and why, on the average, the electrons transfer more energy to 

 the RF field than they take from it. 



