MAGNETRON AS GENERATOR OF CENTIMETER WAVES 255 



and current it is necessary to apply a voltage which on no load would rise 

 to a value considerably higher than the operating value, the rate at which 

 the voltage passes through the range of possible operating values and the 

 relation of this rate to that of RF buildup are extremely important. Should 

 the pulse voltage rise so rapidly as to pass through the region where oscilla- 

 tion is possible before the RF oscillation can build up and cause the mag- 

 netron to pass current, which by modulator regulation keeps the DC voltage 

 from rising further, the magnetron fails to start. Clearly, the more rapid 

 the oscillation buildup the more rapid a voltage rise is permissible. Con- 

 versely, for a given rate of DC voltage rise, failure to start should appear at 

 greater load and longer wavelength as relation (48) impUes. Experience 

 has corroborated both of these conclusions. It is also clear by equation 

 (48) that the equahzation of loading of the doublet modes of the same 

 periodicity, which is achieved by proper location of strap asymmetries, 

 equalizes their starting times and makes possible interference with t mode 

 starting less likely. 



When oscillation in the tt mode fails, the magnetron may fail to oscillate 

 at all or may oscillate in another mode for which the operating voltage in a 

 harmonic is higher than but close to that of the w mode. In this case, as 



has been seen, .—rn > ^;^ , and the Hartree Une of the "second" or "primed" 



\ k' \ N/2 



mode lies just above that of the it mode. This case in which oscillation in 



the TT mode is skipped for oscillation in another mode represents the most 



common type of "moding" encountered in pulsed magnetrons. If, on the 



lirf lirf 

 other hand, , — Vi < — ?- , and the Hartree line of the harmonic of the "sec- 



\ k' \ N/2 



ond" mode lies just below that of the t mode, as in Fig. 16, the magnetron 

 is observed to oscillate first in the "second" mode before oscillation at low 

 currents in the tt mode commences. When the mode driven during the 

 interval at the top of the pulse is the t mode, oscillation in the "second" 

 mode occurs only momentarily on the rise and fall of each pulse as the vol- 

 tage passes through the range of operating values for this mode. 



The starting behavior of pulsed magnetrons may be shown by the so- 

 called dynamic performance chart or V-I plot on which the course in time 

 of the voltage and current is shown. In Fig. 38 are shown three V-I plots 

 of this type. The initial current rise is the charging current of the cathode 

 to anode capacitance. The current rise when oscillation commences is very 

 rapid and is shown as a dashed line. After remaining for the major part 

 of the pulse at the operating point, indicated in Fig. 38 by a large dot, the 

 current and voltage fall during which they follow closely a constant B Une 

 of the static performance chart (see Fig. 17). 



In Fig. 38 (a) is shown the dynamic V-I plot for normal operation in the 



