TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 1291 



that tube life in excess of 10,000 hours was possible with such a cathode. 

 Moreover, an electron gun of the required convergence (about 13° half 

 angle) could be designed using standard techniques. 



The various dimensions, parameters, voltages and currents involved 

 in the design of the MI789 are summarized in Table I. For the sake of 

 completeness, some rf performance data are also included. 



III. DESCRIPTION OF THE TUBE 



3.1 General Description 



This section describes the mechanical structure of the MI789 and 

 presents some performance data closely associated with particular 

 portions of the tube. The overall rf performance is reserved for considera- 

 tion in the next section. In the MI789 we have tried to achieve a design 

 which could be easily modified for experimental purposes and which 

 would also be adapted to quantity production. To assist in obtaining low 

 gas pressure, a rather "open" structure is used, thereby minimizing the 

 pumping impedance. In addition, all parts are designed to withstand 

 comparatively high temperatures during outgassing, both when the tube 

 is pumped and, in the case of the helix and gun assemblies, during a 

 vacuum firing treatment prior to final assembly. Fig. 3 shows an MI789 

 and its subassemblies. Fig. 4 shows a simplified drawing of the whole 

 tube and Fig. 5 shows how the tube is mounted with respect to the perma- 

 nent magnet circuit and to the waveguides. The permanent magnets are 

 shown schematically in Fig. 5. In actual practice they are shaped so as 

 to produce a uniform field between the pole pieces. The means of doing 

 this was discussed by M. S. Glass at the Second Annual Meeting of the 

 I.R.E. Professional Group on Electron Devices, Washington, D. C., 

 October 26, 1956. 



Control of Positive Ions 



Our experience with previous TWT's has indicated that an improve- 

 ment in life by as much as a factor of ten is obtained by arranging the 

 dc potential profile so that positive ion bombardment of the cathode is 

 minimized. This improvement has been observed even in tubes in which 

 all reasonable steps have been taken toward minimizing the residual 

 gas pressure. From Table I it is seen that the relative values of ac- 

 celerator, helix, and collector voltage are arranged to drain positive ions 

 formed in the helix region toward the collector. These ions are thereby 

 kept from reaching the cathode. Spurious ion modulation which can 

 result from accumulation of ions in the helix is also prevented.^ 



