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BELL SYSTEM TECHNICAL JOURNAL 



In typical traveling-wave tubes the lossy section usually has a length 

 such that CV is j or less. In Fig. 9.8 the loss in db in going through the lossy 

 section, 20 logio | Fi/Fi | , has been plotted vs. CX for QC - 0, .25, .5 for 

 the range CV = to CN = .5. 



We see that, for low space charge, increasing the length of a drift space 

 increases the loss. For higher space charge it may either increase or decrease 

 the loss. It is not clear that the periodic behavior characteristic of the curves 

 for QC = 0.5 and 1, for instance, will obtain for a drift space with tapered 

 loss at each end. The calculations may also be considerably in error for 

 broad electron beams (7a large). The electric field pattern in the helLx differs 



-2 



0.2 03 



CN 



Fig. 9.8 — Suppose that we break the circuit and insert a drift tube of length measured 

 by CN in terms of the traveling-wave tube C and N . Assume an increasing wave only 

 before the drift tube. The increasing wave beyond the drift tube will have a level with 

 respect to the incident increasing wave as shown by the ordinate. Here d = and h is 

 chosen to maximize X\ . 



from that in the drift space. In the case of broad electron beams this may 

 result in the excitation in the drift space of several different space charge 

 waves having different field patterns and different propagation constants. 



A suggestion has been made that the introduction of loss itself has a bad 

 effect. The only thing that affects the electrons is an electric field. Unpub- 

 lished measurements made by Cutler mode by moving a probe along a helix 

 indicate that in typical short high-loss sections the electric field of the 

 helix is essentially zero. Hence, except for a short distance at the ends, 

 such lossy sections should act simply as drift spaces. 



9.8 Overall Behavior of Tubes 



The material of Chapters VTII and IX is useful in designing traveling- 

 wave tubes. Prediction of the performance of a given tube over a wide range 

 of voltage and current is quite a different matter. For instance, in order to 

 predict gain for voltage or current ranges for which the gain is small, the 



