The Nature of Power Saturation in 

 Traveling Wave Tubes 



By C. C. CUTLER 



(Manuscript received February 2, 1956) 



The non-linear operating characteristics of a traveling wave tube have 

 been studied using a tube scaled to low frequency and large size. Measure- 

 ments of electron beam velocity and current as a function of RF phase and 

 amplitude show the mechanism of power saturation. 



The most important conclusions are: 



I. There is an optimum set of parameters (QC = 0.2 and yro = 0.6) 

 giving the greatest efficiency. 



II. There is a best value of the gain parameter "C" which leads to a best 

 efficiency of about 38 per cent. 



III. A picture of the actual spent beam modidation is now available 

 which shows the factors contributing to traveling wave tube power saturation. 



INTRODUCTION 



The highest possible efficiency of the travehng wave tube has been 

 estimated from many different points of view. In his first paper on the 

 subject^ J. R. Pierce showed that according to small signal theory, when 

 the dc beam current reaches 100 per cent modulation an efficiencj^ of 



, = § (1) 



is indicated,* and thus the actual efficiency might be limited to some- 

 thing like this value. Upon later consideration" he concluded that the ac 

 convection current could be twice the dc current and that one might 

 expect an efficiency of 



r) = 2C (2) 



He also considered the effects of space charge, and concluded on the 



* Symbols are consistent with Reference 2 and are listed at the end of this 

 paper. 



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