256 



BELL SYSTEM TECHNICAL JOURNAL 



w mode. In Fig. 38 (b) is shown a dynamic plot after the voltage control 

 has been raised above the point of tt mode failure. Here the magnetron 

 does not oscillate at all. As seen in Fig. 38 (c), further increase of the 

 voltage control of the pulser makes possible oscillation in the ^ = — 5 

 (p = —1) harmonic of the n = 3 mode (N = 8). In both (b) and (c) of 

 Fig. 38 it is of interest to note how the magnetron tries to oscillate in the 

 X mode as the voltage at the end of the pulse falls through the range of per- 

 missible values. These attempts at oscillation are indicated by the mag- 

 netron drawing in this region small amounts of current which vary from 

 pulse to pulse. 



Fig. 38. — Three so-called dynamic V-I plots or dynamic performance charts illustrating 

 the mode skip phenomenon. The plots are copies of presentations obtained with an oscillo- 

 scope whose vertical deflection is proportional to DC voltage and whose horizontal deflec- 

 tion is proportional to DC current. On any plot the heaviness of the Unes is roughly 

 inversely proportional to the rate at which the coordinates are traversed, and the large dot 

 at the upper extremities represents the operating point at the top of the pulse, (a) shows 

 shows normal operation in the ir mode, (b) shows failure to oscillate in any mode, the ir 

 mode being skipped, (c) shows oscillation in a harmonic of a mode of smaller n. 



10.7 Magnetron Cathodes: One important component part of the magne- 

 tron oscillator which to this point has not been discussed in detail, but which 

 has been assumed present and operating satisfactorily, is the cathode. Its 

 duty is to supply the electrons which serve as the intermediaries between the 

 DC and RF fields. In terms of the usual requirements of vacuum tube 

 cathodes, the number of electrons demanded of a magnetron cathode is little 

 short of prodigious. Magnetron cathodes may be required to deliver current 

 densities of the order of 50 amperes per square centimeter as contrasted 

 with the 0.5 amperes per square centimeter emitted by oxide cathodes in 

 high vacuum tubes normally. 



How oxide surfaces are capable of emitting such enormous currents is 



