328 BELL SYSTEM TECHNICAL JOURNAL 



increased, the decrease in RF voltage in the tt mode places that mode at a 

 disadvantage in competition with the n = 1 mode. 



As a result of these considerations, a new resonator system was designed 

 in which the electronic conductance at the normal operating point was to be 

 two thirds of that in the first design. To maintain the pulling figure invari- 

 ant, it was necessary to reduce the total resonator capacitance. Although all 

 of this capacitance could not be removed from the straps without reducing 

 the mode frequency separation too drastically, a good proportion of it 

 could be. From this, one might expect a gain in unloaded Q since the straps 

 are the lossiest part of the circuit. 



The new resonator system is more satisfactory than the old. Mechani- 

 cally, it is neater in that the outer strap does not extend into the holes of 

 the hole and slot resonators. A definite increase in over-all efficiency at- 

 tributable to a greater Qo was observed. The mode separation is 17 per 

 cent, much greater than the expected value. An analysis of data on the 

 n = 8, 7, and 6 mode wave lengths, by means of equivalent circuit theory, 

 indicated that this is due to the straps being effectively shorter although 

 their physical length is unchanged in the new design. This is plausible, 

 however, for in the new design the outer strap is connected at a higher 

 voltage point along the resonator. 



The cathode structure of the 4J50, 4J52, and 4J78 magnetrons represents 

 a radical departure from previous designs. It was desirable from the 

 production standpoint to be able to build the cathode structure completely 

 as a subassembly before mounting it in the magnetron. This entailed 

 making holes in the magnet pole pieces large enough for the cathode to 

 pass through ; these holes were, in fact, made of the same diameter as the 

 anode (0.319 in.). Since no radial cathode support leads were required, 

 it was possible to reduce the end spaces to a height of 0.065 in., making the 

 magnet pole gap 0.380 in. It was recognized that the large hole to gap 

 ratio would result in two bad effects: first, a loss of magnetic field and, second, 

 an antibarreling of the magnetic field which results in an axial force acting 

 on the electrons, directed away from the center of the interaction space. 

 Both of these difficulties were surmounted by the use of cathode end struc- 

 tures made of high Curie temperature permendur (50 per cent iron, 50 per 

 cent cobalt). 



The permendur pieces, toroidal in shape, are mounted at the two ends 

 of the cathode (see Figs. 75 and 78). Their cross section and location, 

 necessaty for a nearly uniform field over 80 per cent of the gap and a focusing 

 field over the remainder, were determined in electrolytic tank experiments. 

 Since the permendur pieces fill up a part of the hole in the magnet pole piece 

 and have a separation less than the pole gap, they contribute substantially 



