MAGNETRON AS GENERATOR OF CENTIMETER WAVES 



185 



to understand its fundamental electronic mechanism. As in any oscillator, 

 the criterion for oscillation is that more energy shall be transferred to the 

 RF field by electrons driven against it than is taken from the RF field by 

 electrons accelerated by it. This can be accomplished in the traveling wave 



t + 1 /8T 

 t + 1 /4 T 



t + 2/4 T 



t + 3/4 T 



t + 4 /4 T 



t+ 5/4 T 



t + 6/4 T 



t + 7/4 T 



t -t- 8/4 T 



t + 9 /4 T 



t -t- 1 /4 T 



M 



W 



1 



'A 



^^ 



''A 







/A/ 



2Z 



z\ r^7\ r~::\ f ^ 



--y-^ 



\:^J ^"cl^ 





\ I ^v_^ 





/ 



\ / ^y d" 



^ 



j^-x_^ — ^^ 



\ /~^\_^^= ^ 



^Vr\ ^^ \=z/ 



1 \ 



\ \ / V / r^ ; 



^,jrj — ^^^rrxi 



\S-^' — ^v ^ =^ 



^^i\ / — y^T^\^ 



-6 



/ 

 -14 



Fig. 11. — A plot similar to those of Figs. 9 and 10 for the rotating wave of anode potential 

 of periodicity » = 2 in a magnetron having eight resonators [see equation (13) in the text]. 

 The field at the instant / + |r is plotted as a dashed line to show that the traveling wave 

 does not preserve its shape at all instants. Whereas the wave travels in one direction with 



the angular velocity -^p, electrons which travel with velocities —z-, -777, -77, • • ■ in the 

 Z L lU 10 



. , 1 . ■ 27r/ 27r/ 

 same directioii or with velocities —7-, -77, 



6 14 



in the opposite direction interact favoral)Iy 



with the RF field. Directions of electron motion must now be distinguished. Electrons 

 whose velocity is opposed to that of the rotating field are said to be driving a "reverse" mode. 



magnetron oscillator only if the mean angular velocity of the electrons is 

 such as to make them pass successive gaps in the anode at very nearly the 

 same phase in the cycle of the RF field across the gaps. Then it is possible 

 for an electron which leaves the cathode in such phase as to oppose the 

 tangential component of the RF field across one anode gap, to continue to 



