SPARK GAP SlVfTCriES FOR RADAR 589 



If the switch voltage has been increased to a value near the maximum 

 operating voltage, the voltage-time characteristic shown in Fig. 17 (b) 

 results. Exactly the same sequence occurs as before. However, if the vol- 

 tage be slightly increased above the value shown, the gaps can break down 

 spontaneously during the network charging cycle and before the application 

 of the trigger pulse, even though the value of A is some 20% greater than the 

 charging voltage applied to the gap. This is the expected effect of spark 

 formation time on minimum breakdown voltage since the rate of rise of 

 trigger voltage is far higher than that of the network charging voltage. 

 When spontaneous breakdow^n occurs, because of circuit conditions, both the 

 rate of rise of the voltage of the network charging cycle and its peak value 

 are increased. Since the switch voltage arrives at a higher value in a shorter 

 time, spontaneous breakdown is most likely to occur again. The effect is 

 cumulative so that, after a few increasingly frequent cycles, an arc is 

 estabUshed. It is clear that this arcing must never be allowed to occur in 

 the operating range. 



These characteristics were taken while using a current pulse of 0.75 /xs 

 duration at a repetition rate of 1000 per second and a 30 ohm resistance load. 

 This produced a peak current at the maximum operating voltage closely 

 closely equal to the switch voltage divided by twice the resistance load, or 

 about 100 amperes. Under these conditions, due to the relatively low pulse 

 repetition rate, there is little residual ionization in the gaps at the time of the 

 next pulse, so that the gaps have closely recovered their maximum break- 

 down voltage. However, as the pulse rate is increased, thus decreasing the 

 time between pulses, the value of switch voltage at which the gaps break 

 down spontaneously is found to decrease due to residual ionization. Thus 

 the maximum operating voltage is a function of the pulse repetition rate. 



The decrease of the maximum operating voltage as a function of pulse 

 rate, for these tubes, is shown in Fig. 18 for a variety of pulsing conditions. 



Cur\'e 2 was obtained with the 0.75 jus pulse and a 30-ohm load. It will 

 be observed that the maximum operating voltage decreases with pulse rate 

 in the expected manner. 



If the peak current of the pulse be decreased, fewer ions are produced in 

 the spark and so at any given time after the pulse one w^ould expect less 

 residual ionization in the gaps. Cur\'e 1 was obtained by keeping the pulse 

 duration the same as before but increasing the load resistance to 55 ohms. 

 Thus the current at a given switch voltage was reduced to 30/55 of its former 

 value. It will be seen that, as predicted, the drop of maximum operating 

 voltage with increased pulse repetition rate is less. 



Conversely, if the current is increased the opposite effect is produced. 

 Cun.-e 3 was obtained by decreasing the load resistance to 15 ohms while 

 keeping the pulse duration constant. This gives twice the peak current at 



