WAVE PICTURE OF MICROWAVE TUBES 1355 



In the case of the space-charge-wave structure of Fig. 7, the stop band 

 occurs for conditions near that in which for both sections A and B the 

 section lengths La and Lb are such that 



2 ^ L,, = T 



Uo 



2'^Ls = T 



Here Wy.i and w^s are the effective plasma frequencies for sections A and 

 B. 



A structure such as that of Fig. 7 can be interposed between input and 

 output circuits, such as resonant cavities, to give a space-charge-wave 

 amplifier dependent for its action on the growing wave of the pair. 



THE TRAVELING-WAVE TUBE 



In the space-charge-wave tube, the two waves which are coupled to- 

 gether ha\'e different \'elocities, just as the forward and backward waves 

 on an electron stream have different velocities. Hence, they can be 

 coupled strongly only through the use of some periodic structure in which 

 the period is related to the difference in phase constants of the two waves. 



In a traveling-wave tube we can hstve coupling between a space- 

 charge wave and a wave traveling on a circuit, and both of the waves can 

 have velocities which are nearly or exactly the same. 



Here we must consider two different cases. If both of the coupled waves 

 carry power in the same direction (that is, if the power is positive for 

 both, or negative for both), coupling cannot result in a stop band, but 

 only in transfer of po^^'er between one wave and the other. In order to 

 ha\'e a stop band, power which we try to send in on one wave must come 

 back to us on the other. Hence, to produce a stop band and gaining 

 waves, the two coupled waves must carry powers \\ith opposite signs. 



A traveling-wave tube can consist of a helix of ^\•ire, which can sup- 

 port a slow electromagnetic wave, surrounding an electron beam, as 

 sho^^^l in Fig. 9. 



I TOUT 



Fig. 9 — The vital elements of a traveling-wave amplifier are an electron 

 stream and a slow-wave circuit which may be a helix surrounding the electron 

 stream. 



