856 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



EFFECT OF BEAM SIZE 



In small signal operation, decreasing the beam radius below that which 

 assures a constant circuit field throughout the beam has no effect except 

 that accounted for by its effect on QC. Fig. 9 shows that for large signals, 

 however, it has a pronounced effect. When the beam is made smaller 

 (with QC maintained by changing frequency and beam current), the 

 slowed up tail is formed at a much lower signal level (not shown), by a 

 very few electrons which begin to collect in the accelerating region before 

 the beam is strongly modulated. As the level is increased, the current is 

 redistributed, more going into the tail without much alteration in the 

 shape of the velocity pattern, and with no strong bunching at any part 

 of the curve. This result is exaggerated in Fig. 9(c) by measuring with a 



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240 180 120 60 60 120 



RELATIVE PHASE IN DEGREES 



180 



240 



Fig. 9 — Curves of current and velocity as influenced by yr^ . Space charge 

 becomes a very potent factor near overload, especially when the beam is small. 

 For this run QC = 0.34 and 6 = 1.0. 



