344 BELL SYSTEM TECHNICAL JOURNAL 



length makes the arcing much worse; in the 725 A for example, changing 

 the pulse length from 1 /xs to 2 /zs may increase the arcing rate by a factor 

 of 10 to 20. A decrease in recurrence rates with some types of cathodes 

 also causes a noticeable increase in the rate of arcing. 



In the great majority of cases, magnetron cathodes have employed coat- 

 ings of the ordinar}^ strontium and barium carbonates as the source of 

 primary and secondary electrons. All-metal, secondary emitting cathodes 

 with primary emitting, starter cathodes have been tried in some labora- 

 tories with some success but have not yet come into general use. The efforts 

 at the Bell Laboratories have been directed toward developing and im- 

 proving the oxide coated cathode along lines making it more nearly possible 

 to satisfy requirements (3), (4), (5), and (6) listed in Section 10.7 Magne- 

 tron Cathodes of PART I without impairing the ability to meet require- 

 ments (1) and (2). 



Most of the cathode developments were made in connection with the 725A. 

 This magnetron served as a convenient "laboratory" or "proving ground" 

 for magnetron cathode studies. Its cathode is small enough and the de- 

 mands made upon it stringent enough to make apparent any cathode weak- 

 nesses and any improvements which may be made. A large number of life 

 racks were kept in constant operation for 7 25 A magnetron life tests over a 

 long period of time under various conditions in which the cathodes were 

 generally run to destruction. 



Attempts were made to make the data reproducible by controlling the 

 activation, by controlling the initial break-in of the operating magnetron, 

 and by devising means of quantitatively determining some measure of the 

 adequacy of any given cathode. The last of these items involved the de- 

 velopment of an automatic counter which registers the number of arcs or 

 bursts of arcs which cause the current to exceed a predetermined value. By 

 recording the accumulated number of arcs at intervals throughout the life 

 of the magnetron, it is possible to get a picture of the arcing pattern with 

 life. Such counters have done much to put the life testing and initial break- 

 in of magnetrons on a semi-quantitative basis. The smoothed curves shown 

 in Fig. 80 were obtained through the use of these counters. 



Early attempts at building a cathode upon which sufficient active material 

 may be held, made at both the M. I. T. Radiation Laboratory and the BeU 

 Laboratories, involved winding coated cathodes with nickel wire to provide 

 a reservoir of active material under the wires. In this manner, the material 

 was protected from direct arcing effects, allowing the active material to 

 migrate slowly to the outside surface of the wires. This general idea was 

 greatly improved upon when the wire winding was replaced by a woven nickel 

 mesh into the interstices of which the active material was packed. The 



