1300 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



Meaiirements of beam interception as a function of magnetic flux 

 density are sfiown for several beam currents in Fig. 1 1 . These measure- 

 ments were obtained without any rf input to the TWT. An interesting 

 way of normalizing these data is shown in Fig. 12. Here the magnetic 



ACCELERATOR--. , / 



/ 

 / 



CATHODE ^ / 



^ 



MAGNET 

 POLE PIECE 



, HELIX CONNECTOR 



^ 



_fou tFffWwi)' 



BEAM- 

 FORMING 

 ELECTRODE 



in 



\///^////////A 



MAGNETIC 

 M SHIELD 



-0.4 0.4 0.8 1,2 



DISTANCE FROM CATHODE IN INCHES 



1.6 



Fig. 10 — Variation in magnetic flux density as a function of distance from the 

 cathode. A schematic representation of the gun electrodes and of the magnetic 

 parts which have been used to control the flux is also shown. All the elements in- 

 side the tube are non-magnetic so that the flux density variation is determined 

 entirely by magnetic parts external to the tube envelope. The flux density at the 

 cathode is built up (i.e., the step is put into the curve) by having the magnetic 

 shield end near the cathode. The flux which leaves the shield at this point increases 

 the flux density at the cathode over what it would be if the shield extended well 

 behind the cathode. 



