MAGNETRON AS GENERATOR OF CENTIMETER WAVES 329 



to the magnetic lield in the anode-cathode region. The effective gap is 

 reduced from 0.380 in. to 0.340 in. by their presence, resulting in about a 

 20 per cent decrease in magnet weight. While their primary- function is 

 magnetic, the permendur pieces also serve as normal cathode end disks pre- 

 venting electrons from reaching the pole pieces. Their smooth contour 

 gives a good DC voltage breakdown condition between anode and cathode. 

 Finally, they are mounted in such a way that there is some thermal isolation 

 from the cathode and that the migration path from cathode to external 

 surface of the permendur is quite long. These features discourage electron 

 emission from the cathode end structures. 



Almost any degree of cooling may be obtained with an axially mounted 

 cathode without the attendant disadvantages of the heavy tungsten leads 

 which fill up the end spaces of a magnetron having radial cathode mounting. 

 An upper limit to the cooling is set by the fact that the cathode must be 

 raised to 1050°C in activation, using a heater which can be contained within 

 the cathode sleeve without encroaching too much upon its wall thickness. 

 The high temperature needed during activation sets certain limits upon the 

 materials which may be used in the cathode structure. Since the cathode is 

 mounted from one end only (this being dictated by assembly considera- 

 tions), mechanical strength is exceedingly important. A cathode, once 

 off center, is subjected to magnetic forces on the permendur ends which 

 tend to pull it further off center. Fortimately, measurement at the mag- 

 netic fields used in normal operation shows that these forces are only of the 

 order of one pound for a cathode position 0.015 in. off the axis, increasing 

 to about 3.5 pounds when the permendur is in contact with the wall. 



The first cathode surfaces used in these magnetrons were of the "mesh" 

 type. Later, the newly developed, sintered nickel matrix type was used. 

 More recently, considerations of strength have led to the introduction of 

 molybdenum cathodes. The cathode assembly is brazed into a hollow 

 metal cone in the base of which is a receptacle type heater and cathode 

 connection. Because of the length of the supporting structure and the 

 relatively high temperature at which it operates, there is considerable 

 expansion and resultant motion of the cathode in the axial direction. The 

 cathode is offset when cold to correct for this expansion. Performance is 

 not very sensitive to cathode location, but in extreme cases of misalignment 

 moding difficulties may be aggravated. The cathode structure is to be 

 seen in Fig. 78 and is shown mounted in the magnetron in Fig. 75. Atten- 

 tion should be called to the external cathode input lead which is of hea\y 

 glass and Kovar construction sufficiently rugged to make unnecessary any 

 protective housing. 



In the wave guide output designed for the 4J50 magnetron, the necessary 



