TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 



1299 



rangement did result in the best beam transmission to the collector. 

 We later discovered, however, that the noise on the electron stream be- 

 came extremely high when there was no magnetic flux at the cathode. 

 This effect will be discussed further in Section IV. We found that by 

 having a flux density of about 20 gauss at the cathode, the noise figure 

 could be considerably reduced with the only penalty being a slight in- 

 crease in interception on the helix. The penalt^y results from the fact that 

 the flux linking the cathode causes a reduction in the angular velocity of 

 the electrons in the helix region (from Busch's theorem), and this in 

 turn diminishes the magnetic focusing force. 



Fig. 10 shows the distribution of axial magnetic field in the gun region. 

 The curve represents a compromise between that which gives best fo- 

 cusing (zero flux density at the cathode) and that which gives best 

 noise performance (about 25 gauss flux density at the cathode). This 

 flux density variation was arrived at by empirical methods. 



CATHODE 



SUPPORT LEGS 



THREE AT 120° 



SEPARATION 



HEATER T 



HEATER LEAD 

 INSULATOR 



CATHODE 



INNER CATHODE 

 - ALIGNMENT 

 CYLINDER 



METAL TABS 



TO HOLD 



HEATER IN 



PLACE 



Fig. 9 — The cathode subassembly. In this unit the cathode is connected to 

 the inner alignment cylinder by three legs. These legs are first welded to the cath- 

 ode and then oven brazed to the alignment cylinder. During the brazing, a jig 

 holds the cathode accurately concentric with this cylinder. The cathode is then 

 coated and the unit is ready for assembly into the gun. The heater power required 

 to raise the cathode to its operating temperature of 720°C is about si.x watts. 



