SPARK GAP SWITCHES FOR RADAR 583 



with iron-sponge mercury cathodes were developed to the manufacturing 

 stage, as discussed in Il-(f). 



In addition to being capable of switching higher peak powers than alumi- 

 num cathode gaps, the mercury cathode gaps can be designed to have supe- 

 rior operating characteristics. Through the use of small radius anodes not 

 possible with the aluminum cathode gaps, a wider operating range and much 

 less time "jitter" can be attained. The small anodes build up corona at 

 voltages less than those of breakdown, thus furnishing radiation prior to 

 breakdown. For special applications, gaps have been developed having a 

 range approaching 3 to 1 in a two-gap circuit, capable of switching 10 mega- 

 watts peak power, for many hundreds of hours, and having a time "jitter" 

 of less than 0.02 microseconds at the operating voltage. 2- ^ 



(d) Starting and Operating Characteristics 



It has already been stated in IT(a) that starting and operating charac- 

 teristics of series gaps cannot be interpreted simply because, under the 

 circuit conditions of rapidly varying voltage, the breakdown voltage of a 

 spark gap is not singly valued. Because of spark formation time the 

 minimum voltage at which a spark gap will break down increases as the rate 

 of rise of the voltage across it increases. Further, due to spark delay time, 

 the voltage across the gap at breakdown is usually still higher than this 

 minimum value. It is therefore impossible to designate a unique 

 breakdown voltage of a spark gap when the voltage across it is increasing 

 with time. It is, however, possible to lind a practical minimum and maxi- 

 mum breakdown voltage for a particular rate of rise of voltage. The differ- 

 ence between this maximum and minimum value is a measure of the maxi- 

 mum spark delay time. It is for the purpose of reducing this spark delay 

 time that corona points (or radium) are introduced, and it will be shown that 

 the value of both spark delay time and spark formation time have an im- 

 portant bearing on the operational characteristics of fixed gaps. 



In addition to rate of voltage rise, the breakdown voltage of a spark gap 

 depends on the amount of ionization in the gap due to a previous spark. 

 When a spark discharge stops, a column of highly ionized gas is left in the 

 gap. Although this column is rapidly de-ionized by recombination and 

 diffusion of ions, a lower breakdown voltage is found for many micro- 

 seconds in consequence of this residual ionization. The minimum value 

 of the breakdown voltage of the gap is therefore a function of the time 



2 F. S. Goucher, J. R. Haynes and E. J. Ryder, High Power Series Gaps Having Sin- 

 tered Iron Sponge-Mercury Cathode, P.B. 19640, U. S. Department of Commerce, Office 

 of the Pul)lication Board. 



3 J. R. Dillinger, Operation of Sintered Iron Sponge-Mercurv Cathode Type Series 

 Gaps at S.C.I.. .\.E.W. and 5 Microsecond Conditions, P.B. 13270, U. S. Department of 

 Commerce, Office of the Puljlication Board. 



