TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 1295 



The effect that ions can have on cathode life was clearly demonstrated 

 in a TWT which was in many aspects a prototype of the MI789. This 

 tube operated with the accelerator, helix and collector at successively 

 higher voltages, with consequent ion draining toward the cathode. Severe 

 ion bombardment of the cathode brought about failure of most of 

 these tubes in from 500 to 2,000 hours. In contrast to this the average 

 life of the M1789 is in excess of 10,000 hours in spite of a cathode current 

 density about twice that in the prototype tube. Moreover, failure of the 

 M1789 comes about from exhaustion of coating material rather than as a 

 result of ion bombardment. During the course of the work of the proto- 

 type tube, an experiment was performed to determine how much the 

 ion bombardment would be affected by changing the potential dif- 

 ference between tube electrodes. In this experiment a small hole was 

 drilled in the center of the cathode and an ion current monitoring elec- 

 trode placed behind it. The ion monitor current was then investigated as 

 a function of electrode voltages. Fig. 6 shows the results. We see that 

 comparatively small potential differences are adequate to control the 

 flow of positive ions. 



3.2 The Electron Gun and Electron Beam Focusing 



The electron gun used in the MI789 is a converging Pierce gun. The 

 values of the gun parameters are summarized in Table I. Included are 

 both the original parameters introduced by Pierce as well as those defined 

 in a recent paper by Danielson, Rosenfeld and Saloom- in which the 

 effects of thermal velocities are considered. Fig. 7 shows a drawing of 

 the electrically significant contours of the J\II789 gun. Fig. 8 shows the 

 completed electron gun assembly. The method of constructing the gun is 

 a modification of a procedure used in oscilloscope and television picture 

 tubes. The electrodes are drawn parts made of molybdenum or, in the 

 case of the cathode, of nickel. They are supported by rods which are in 

 turn svipported from a ceramic platform to which these rods are glazed. 

 The whole gun structure is supported from the end of the helix by the 

 helix connector detail. Since this part must operate at helix potential, 

 it is insulated from the remainder of the gun by a ceramic cylinder which 

 is glazed both to it and to the accelerator. 



To obtain good focusing, the cathode must be accurately aligned with 

 respect to the other electrodes. However, it must be omitted from the 

 gun during the glazing process and during a subsequent vacuum out- 

 gassing because the cathode coating cannot withstand the temperatures 

 involved. To insure proper placement of the cathode in the gun assembly 



