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for example, took as long as 45 minutes of intense arcing during gradual 

 increase of input voltage and current before satisfactory operation was 

 attained. It seemed unlikely that the arcing during the break-in period 

 could be completely eliminated, but it was found that improvements in 

 cathode construction which made steady operation at higher current 

 possible also reduced the time of initial break-in to attain these conditions. 

 It appeared that the resistance of the cathode coating might be lowered 

 by distributing nickel more uniformly throughout the coating. Accord- 



O 0.025 



O 0.020 



200 400 600 aOO 1000 1200 1400 1600 1800 



AGE IN HOURS 



Fig. 80 — Curves of arcing during life for 725A magnetrons, having various types of 

 cathodes, operating at 5. 7/js pulse duration, 165 pulses per second, and 210 kw. peak power 

 input. It is to be emphasized that these conditions are considerably more stringent than 

 the normal operating conditions. Curve (a) is for a cathode having oxide coating on a 

 nickel wire mesh base; curve (b), metallized coating on a nickel wire mesh base; curve (c), 

 either plain oxide or metallized coating on a sintered, nickel powder, matrix base. 



ingly, mesh cathodes were coated with a mixture of double carbonates in 

 which was distributed a fine nickel powder of high purity having an average 

 particle size of 2 microns. The carbonate particle size is between 1 and 2 

 microns. The amount, found not to be critical, was 55 per cent by weight 

 of the combined dry ingredients of the coating mixture. Although this 

 reduced the amount of active material present on the cathode surface, as 

 seen in Fig. 80 it resulted in a cathode having considerably improved arcing 

 characteristics with considerably longer useful life. This cathode con- 

 struction was adopted finally for the 725A production and was also utilized 

 in the 10 cm. high power magnetrons, the 4J45-47, for operation at 5 jis, 

 pulse duration. 



