572 BELL SYSTEM TECHNICAL JOURNAL 



later section, but, in view of the fact that the gap itself plays an important 

 part in these characteristics, it seems desirable to describe first the gap types 

 with which we have to deal. 



(b) The Hydro gen- Argon Aluminum Cathode Gap 



Following the successful triggering of fixed gaps in air without the use of 

 air blast for their de-ionization, experiments were undertaken with sealed 

 gaps in various gas atmospheres using simple rod electrodes having their 

 axes parallel. A large number of gases were tested and the conclusion 

 reached that hydrogen was the most satisfactory because of its high de-ion- 

 ization rate. With it fewer and wider gaps were required to meet a given 

 pulsing condition. Three 4 mm. gaps in hydrogen at pressures somewhat 

 less than atmospheric were approximately equivalent to the five 0.5 mm. gaps 

 in air already referred to. Thus, from this point of view, the use of hydrogen 

 would very greatly simplify the problem of making practical gaps. 



The spark in hydrogen, particularly with relatively small peak currents, 

 was, however, unsatisfactory in that it terminated in a high-pressure glow 

 with a high cathode drop rather than the low drop required for efficient 

 switching. The addition of about 25% argon corrected this and about this 

 proportion was used successfully in the gaps with which we are concerned in 

 this report. 



Although the required operating conditions were met with this gas 

 mixture, cathode erosion or sputtering was so excessive with all readily 

 available cathode materials that this factor appeared as the chief obstacle in 

 the way of making practical gaps. The sputtered material was deposited on 

 all surfaces in the form of a fine powder which eventually destroyed the 

 insulation, thereby limiting the useful fife of the gaps to a few hours. ^ 



A promising lead was, however, obtained in the case of aluminum cathodes. 

 It was observed that some of the sputtered material deposited on the 

 anodes opposite the cathodes from which it was removed. This deposit 

 was reasonably compact and smooth, which suggested the possibility of 

 reducing by gap design the extent of harmful scattering. This might be 

 achieved by increasing the amount of sputtered cathode material which is 

 deposited on the anode or returned to the cathode within the sparking area. 



The tube, Fig. 6, was an early attempt in this direction. This tube had 

 three 4 mm. gaps between flat electrodes, the cathode surfaces having 

 raised portions to confine the sparking within their areas. The gaps were 



' At about this time we learned that the British had developed sealed gaps triggered 

 by means of an auxiliary electrode and known as "Trigatrons." These were high pressure 

 gaps containing argon with a small amount of ox\-gen to reduce sputtering of the elec- 

 trodes. The life of these gaj)s was determined b}- the time required to clean up this ox}'- 

 gen. Though these were tried it was decided to follow an independent development 

 avoiding if possible all clean up effects. 



