54 



BELL SYSTEM TECHNICAL JOURNAL 



to the coaxial line. The two identical probes pi , and p2 , extending from 

 coaxial lines into the two electron streams at the beginning and end of the 

 central portion of the tube between the two helices were inserted for com- 

 parison of the signal amplitudes at the be;.^innini? and end of the region in 

 which no circuit is present. 



A similar tube containing an output gap in place of the output helix sec- 

 tion is represented in Fig. 2. 



In both cases concentric tubular electron streams originate at the ring- 

 shaped emitting surfaces of the two cathodes at potentials Vi and V2 , pass 

 through their respective control grids and through a common accelerating 

 grid. An axial magnetic field of approximately 700 gauss is applied in order 

 to maintain the definition of the beams. The outer and inner tubular beams 





.\^^^^ ^ kwvvvC- 



( k ^ k t k k-rt Ik k k k k k k I.WVrr-'.W 



l|l|l|l l|l|l|l| 



C, C,- INNER AND OUTER CATHODES 



S S _ II I. II ELECTRON STREAMS 



T -HELIX TERMINATION 



H -INPUT HELIX 



G -OUTPUT GAP 



Fig. 2 — Representation of double-stream amplifier with gap output. 



have mean radii of 0.215" and 0.170" respectively and a wall thickness of 

 0.030" in each case. 



The short sections of helix which are used for input and output are wound 

 of 0.013" diameter molybdenum wire, 44 t.p.i., and mean diameter of 0.500". 

 The axial velocity of signal propagation along this helix is equal to that of 

 54- volt electrons. The helix sections are each 2" long. Ceramic supporting 

 rods on each helix section are sprayed with aquadag, over Ij" of their 

 length on the end nearest the center of the tube, for terminating purposes. 

 The thickness of the spray coating increases toward the center of the tube. 

 The distance between helices is 8.7". 



The gain produced by the electronic interaction depends upon a difference 

 in velocity between the two electron streams. The signal is impressed 

 upon one of the streams by the helix when its velocity is that at which travel- 

 ing wave amplifier interaction between the stream and the helix occurs. It 



