MAGNETRON AS GENERATOR OF CENTIMETER WAVES 



345 



nickel mesh, being spot welded or sintered directly to the nickel base, formed 

 a part of the cathode foundation, helping to reduce the coating resistance. 

 It was found that arcs, rather than striking directly to the active material, 

 tended to strike to the nickel wires. In spite of this fact, sudden bursts of 

 current in arcing would erupt some of the active material from the cathode. 



The mesh used in the 725A cathode is made of 6 mil nickel wire woven 

 with 75 wires to the inch. The radial thickness of the mesh on the cathode 

 is 10 mils. The average amount of active double carbonate mixture which 

 the cathode holds is 23 mg. per sq. cm. The mesh cathode is shown in Fig. 

 79 in three stages of its fabrication. With this cathode, it was possible to 



Fig. 79— 725A cathode construction. Shown are: (1) the heater element; (2) the 

 machined nickel cathode blank; (3) the cathode blank with the nickel wire mesh, welded 

 or sintered in place; (4) the mesh type cathode with oxide coating applied, cut away to 

 show heater element in place; (5) the more recent cathode with sintered, nickel powder, 

 matrix base (uncoated). 



produce 725A magnetrons having a guaranteed life at rated operating 

 conditions of at least 500 hours, the average value being considerably higher. 

 The arcing characteristic of the mesh cathode, excluding the initial break-in 

 period, is shown in Fig. 80. The abrupt failure is typical of magnetron 

 cathodes. At shorter pulse lengths failure occurs considerably later but in 

 the same abrupt fashion. The life curve of the early type of cathode con- 

 sisting of an oxide coated nickel cylinder, if plotted with the data on other 

 cathodes in Fig. 80, would be crowded very close to the axis of zero hours. 

 Even with cathodes of mesh construction, operation of the magnetron 

 was generally inadequate at 5 fis pulse duration. Furthermore, consider- 

 able break-in time was necessary before stable operation at 1 )us pulse dura- 

 tion was achieved. Some of the first mesh cathodes used in the 725A, 



