NATURE OF POWER SATURATION IN TRAVELING WAVE TUBES 855 



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 RELATIVE PHASE IN DEGREES 



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Fig. 8 — A comparison sliowing the effect of the space charge parameter QG 

 1 on the velocity and current at overload. The points represent the disc electrons of 

 { the computations^ of Tien, Walker and Wolontis. For this run 7ro = 0.4 and h is 

 I chosen for maximum X\ . 



I greater reduction of velocity results in a faster and farther retarding of 

 I the current in the spur before the retarded electrons recover velocity in 

 i the accelerating region. Also the larger space charge forces prevent as 

 I tight bunching of the electrons anywhere, so that at overload they are 

 •spread over a much wider phase interval (about 360° for QC = 0.5). 

 ! Space charge also prevents electrons from the forward part of the bunch 

 j from being trapped so that more electrons escape ahead of the decelerat- 

 ' ing field and more current is found in the upper half of the velocity 

 j curve. This very likely is the reason that efficiency decreases when QC 

 \ is increased above about 0.3. 



