TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 



1301 



flux density has been divided by the Brillouin flux density for a beam 

 entirely filling the helix. This quantity is the minimum flux density 

 which could theoretically be used to focus the beam. This normalization 

 tends to bring all of the curves together. Thus we see that, although 

 the conditions in the IMI789 are far from those of ideal Brillouin flow 

 (because of transverse thermal velocities, aberrations in the gun, and 

 magnetic field at the cathode), the concept of the Brillouin flux density 

 still retains meaning, i.e., it appears that the flux density required main- 

 tains a fLxed ratio to the Brillouin value. 



Applying sufficient rf input to the MI789 to drive it into non-linear 

 operation, results in defocusing caused by the high rf fields (both from 

 the helix wave and from space charge) near its output end. Fig. 13 shows 

 how the beam interception for different magnetic flux densities varies as 

 a function of the power output of the TWT. From these curves we see 

 that an output level of five watts can be maintained with about one per 

 cent interception with a flux density of 600 gauss. 



6.( 

 x 



_J 

 m 

 I 4.1 



z 

 o 



a 



LU 



4.0 



3.5 



3.0 



I- 

 z 2.5 



LU 



cr 

 tr 



D 

 U 



2.0 



5 

 < 



LU 

 OD 



U. 

 O 



LU 



1.5 



1.0 



O 0.5 

 tr 



UJ 



Q- n 



200 



300 



400 500 600 



MAGNETIC FLUX DENSITY IN GAUSS 



700 



600 



Fig. 11 — Per cent intercej^tion on the helix as a function of magnetic flux 

 density. These measurements were taken using a precision solenoid to focus the 

 TWT. The component of field perpendicular to the TWT axis was less than 0.1 

 per cent of the longitudinal field. During these measurements there was no rf 

 input to the TWT and there was substantiallj- no (<0.1 ma) interception on the 

 accelerator electrode. 



