182 BELL SYSTEM TECHNICAL JOURNAL 



action of the electrons with the tangential component of a traveling wave 

 whose velocity is approximately equal to the mean translational velocity 

 of the electrons. Later" the role of the radial component of the rotating 

 electric field in keeping the electrons in proper phase was recognized. 

 Magnetrons of wavelength as short as 75 cm., operating at better than 50% 

 efficiency, were built prior to 1940, but performance such as was later to be 

 attained with this type of magnetron at much shorter wavelengths was not 

 attained then, perhaps primarily because of the lack of a good resonator. 

 It was a magnetron of this type which the British brought to the United 

 States in 1940. The British magnetron was a 10 cm. oscillator, intended 

 for pulsed operation, having a tank circuit consisting of eight resonators 

 built into the anode block as shown in Fig. 1.'^ 



3. The Electronic Mechanism 



3.1 Electronic Interaction at Anode Gaps: The electrons in the interaction 

 space of the magnetron oscillator are the agents which transfer energy from 

 the DC field to the RF field. As such, they must move subject to the con- 

 straints imposed by the DC radial electric and DC axial magnetic fields, 

 considering, for the moment, the RF fields to be small. Under these condi- 

 tions, as has been seen for the DC cylindrical magnetron (see Fig. 4 for 

 B > Be), electrons follow^ approximately epicycloidal paths which progress 

 around the cathode. The mean velocity of this progression, that of the 

 center of the rolling circle, depends upon the relative strengths of the electric 

 and magnetic fields [see equation (5) for the plane case]. By proper choice 

 of DC voltage, V, between cathode and anode and of magnetic field, B, 

 the mean angular velocity of the electrons may be set at any desired value. 



The RF electric fields in the interaction space, with which the electrons 



moving as described above must interact, are the electric fields fringing from 



the slots in the anode surface. These fields are provided b}' the X coupled 



oscillating cavities of which the magnetron resonator system is composed. 



As will be discussed in more detail later, such a system of resonators may 



oscillate in a number of different modes in each of which the oscillations in 



adjacent resonators, and thus the fields appearing across adjacent anode 



slots, bear a definite phase relationship. For a system of N resonators it 



will be seen that the phase difference between adjacent resonators ma}- 



2ir . N 



assume the values n — radians, n being the integers 0, 1, 2, • ■ • , — . 



Adopting another point of view, one may consider the potentials placed 

 upon the anode segments by the resonators. The variation of the potential 



1^ F. Herriger and F. Hulster, Zeit. f. Hochfrequenz, 49, 123 (1937). 



^^ The use of such internal resonators is reported in the literature bv N. T. Alekseieff 

 and D. E. Maliaroff, Journ. of Tech. Phys. (U.S.S.R.) 10, 1297 (1940); republished in 

 English, Troc. I.R.E. 32, 136 (1944). A. L. Samuel has obtained U. S. Patent i^ 2,063,342 

 Dec. 8, 1936, for a similar device. 



