594 BELL SYSTEM TECHNICAL JOURNAL 



can be neglected but which can be measured independently and added if so 

 desired. 



It is to be noted that the first or transient term of the formula is unaffected 

 by pulse duration and argon content and depends at least to a first approxi- 

 mation on only the peak current and length of spark. The numerical 

 constant includes the time of this transient, the average gradient during 

 this period, and a factor to reduce the peak current to an average value. 

 The portion of the second or steady state term within the brackets represents 

 the average voltage across a gap when it is highly conducting and is 

 approaching the characteristics of a steady arc. This average voltage is 

 separated into two parts. The first part, 40 volts, is the sum of the cathode 

 and anode drops arising from space charges at the electrodes. The second 

 part is the voltage drop along the positive column which has a pressure 

 dependent uniform gradient and which is of the order of 100 volts per cm. 

 It is only this gradient which is perceptibly altered when argon is added to 

 the hydrogen. 



From this formula it is possible to calculate the switching efficiency for 

 any design of gap and set of pulsing conditions within the specified range of 

 parameters covered by the formula. Calculation shows that with three 

 gaps in series the switching efficiency in all projects was at least -90%, whereas 

 with two gaps in series it was in most cases as high as 96%. 



(f) Development of Fixed Gaps for Manufacture 



The designs of the fixed gaps for manufacture were dictated by the re- 

 quirements of particular modulators. Under the code number of each of the 

 gaps a brief description is given of the electrical and mechanical require- 

 ments which had to be met. 



W.E. 1B22 



The 1B22 fixed gap tube is an aluminum cathode type with a hydrogen- 

 argon filling. An exterior and a cross-sectional view are shown in Fig. 21. 

 This fixed gap tube was developed for the modulator of an airborne radar 

 known initially as ASH and later an AN/APS-4. In this modulator two 

 tubes are used in series to switch a peak power of about 105 kilowatts into a 

 W.E. 725A magnetron. It was desirable that the peak voltage in the modu- 

 lator section be kept fairly low so that the circuit would perform satisfac- 

 torily at high altitude even when the pressurizing container was damaged. 

 Furthermore, the equipment was to be very compact and light in weight. 



In order to meet the requirements of this radar, two tubes were used in 

 series with a peak switching voltage of 4 kilovolts. They were required to 

 pass a current pulse of 67 amperes for 0.75 microseconds at two repetition 



