NATURE OF POWER SATURATION IN TRAVELING WAVE TUBES 859 



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240 180 120 60 60 120 



RELATIVE PHASE IN DEGREES 



180 



240 



Fig. 10 — The influence of beam velocity on ac velocity and current. When the 

 velocity is raised too high, the electrons are not effectively trapped by the wave, 

 and override into the accelerating field. With large QC and/or small 7ro the elec- 

 trons override in any case, and little is gained by increasing h. For this case QG 

 = 0.13 andTfo = 0.21. 



^main effect being to push more electrons forward into the accelerating 

 j region. 



KLECTRIC FIELD IN THE BEAM 



' Besides telling a clear story of the non-linear dynamics of the traveling 

 , wave tube, the foregoing curves contain a lot of information about aver- 

 jage current and velocity distributions. From the current or velocity 

 curves we can in turn deduce the distribution of longitudinal electric 

 field in the beam. Figs. 11(a) and (b) show the instantaneous current as 

 a function of phase, taken from the curves of Figs. 8(a) and (b). The 

 infinite differential in the velocity curve necessarily gives a pole in the 

 charge density (at about 88°). The total charge in the vicinity of the 



