342 BELL SYSTEM TECHNICAL JOURNAL 



presents a lower limit on pulse duration which may be employed and further 

 complicates the cathode cooling problem. 



21. Magnetron Cathodes 



From the start of its work with centimeter wave magnetrons, the Bell 

 Laboratories has been engaged in an extensive program of magnetron 

 cathode testing and design. The range of cathode size is illustrated in 

 Fig. 78. The largest cathode, shown at the extreme left, is that of the 5J26 

 magnetron, operating at about 23 cm., the smallest, that of the 3J21 magne- 

 tron operating near 1 cm. The range of operating conditions to be met, 

 as well as the magnitude of the problem at shorter wavelengths, is illus- 

 trated by these two extremes. The cathode of the 3J21 magnetron, al- 

 though its surface area is only 0.31 sq. cm., is called upon to deliver peak 

 currents comparable to those demanded of the cathode of the 5J26, having 

 a surface area of 17.1 sq. cm. The ratio of current densities is 25 to 1. 

 The peak back bombardment of the two cathodes is comparable. Other 

 cathodes shown in Fig. 78 operate under conditions intermediate between 

 these extremes. Needless to say, the 5J26 cathode is operated quite con- 

 servatively, the 3J21 cathode under extremely severe conditions. 



Little difficulty with cathodes has been experienced in pulsed magne- 

 trons above 10 cm. wavelength. For the most part they are plain, nickel 

 sleeves coated with active material. The highest emission density (10.1 

 amps./cm.2) ^ggd successfully with this type of cathode was in the 

 720A-E, which operated satisfactorily at 1 jis but was not satisfactory 

 at 5 jus. The operation at 5 yus required a modification of the cathode as 

 described. When the simple sprayed cathode was tried in developmental 

 models of the 725A magnetron, the Hfe at 1 ^^s was of order 10 hours. At 

 the high emission density required (approximately 30 amps./cm.'-) the active 

 coating was rapidly lost as a result of arcs, and frequently the nickel support 

 itself was fused and vaporized. 



The problem of arcing to the cathode surface in small magnetrons was 

 soon recognized as the most important cathode problem. It prevented 

 steady operation and hastened destruction of the cathode surface and the 

 end of useful magnetron life. All of these cathodes were found to require 

 an initial break-in period during which the arcing is particularly violent. 

 As the input voltage and current are gradually increased, the violent 

 arcing gradually subsides. The tendency to arc at any time subsequent to 

 this initial period depends on the nature of the cathode surface, but beyond 

 that, it depends also on the operating conditions to which the magnetron 

 is subjected. Increase in power input, either as increased voltage or current, 

 or both, rapidly increases the frequency of arcs. Similarly, increased pulse 



