ELECTRON TUBES FOR A COAXIAL SYSTEM 



1107 



usually associated with high current density tend to shorten the life. Higher 

 electrode temperatures, higher potentials, and the production of more ions 

 are the major items in this category. It is presumed that the shorter life 

 found under these conditions is due to the greater rate of contamination of 

 the cathode by material from the other parts of the tube. Great efforts 

 have been made to find and to use processing techniques which will mini- 

 mize this kind of limitation and to introduce constituents into the cathode 

 which will counteract such deterioration. The situation at the time the L3 



1200 

 1100 

 1000 

 900 

 800 

 700 

 600 

 500 



ft 



2 400 



300 



O 200 

 u. 



100 



12 3 4 5 6 7 8 



GRID-TO -CATHODE SPACING "a" IN CENTIMETERS 



Fig. 2 — Figure-of-merit vs. grid-to-cathode spacing. 



9 10x10"^ 



tubes were being developed was that 50 MA/cm^ was as high a current 

 density as seemed to be consistent with the long life required. 



It is apparent from the curves in Fig. 2 that the figure of merit increases 

 rapidly as the grid-cathode spacing "a'' is reduced. The limitation here is 

 mechanical and manifests itself in two ways. One is the practical difficulty 

 of spacing the parts so closely with sufficient accuracy. The other is the 

 problems associated with fabricating grids wound with wire of small enough 

 diameter to make effective use of the close grid-cathode spacing. This 

 part of the subject will be discussed in detail later. It is one of the most 

 important aspects of the design of the L3 tubes. 



