Early Types of Underwater Spark Gaps 



The first experiments with underwater sparks as a 

 source of sound utilized special types of automotive and 

 aircraft spark plugs. Since spark plugs were not designed 

 for use underwater, in general they were unplated and 

 rusted very quickly. Several better types were found that 

 were either chromium or cadmium plated. The types 

 with multiple discharge points, such as the aircraft variety 

 designed for igniting jet fuel, proved most satisfactory. 

 In general it was found that the porcelain insulation near 

 the firing points blew off on the first firing but the plugs 

 continued to function. 



It was concluded very early in the experimental 

 program that special spark gaps would have to be designed 

 to handle any substantial power level. For energy levels 

 below about 100 watt-seconds, several types of plugs proved 

 satisfactory. To obtain greater power, attempts were made 

 to operate plugs in various series and parallel combinations. 

 Such arrangements usually proved unsatisfactory since 

 the plugs would seldom fire simultaneously. Firing would 

 be random and almost unpredictable due to non-uniform 

 point burning. Various modes of insulating and encapsula- 

 ting the plugs were tried. A few of the combinations are 

 shown in figure 3. 



Energy Transfer 



The sudden discharge of a large amount of stored energy 

 into a virtual short circuit is a great strain on any type of 

 electrical switch. A literature search revealed that many 

 experimenters are still using a three -ball open air gap for 

 the energy transfer. This method may result in extremely 

 low efficiency. Hydrogen thyratrons are fairly effective but 

 are short-lived and have associated "flash-back" troubles. 



From the beginning of the underwater spark studies at 

 NEL, specially designed high-vacuum switches have been used 

 for the energy transfer function. Switching the high voltages 

 and current in a vacuum solves many problems and is prob- 

 ably the most practical way of accomplishing the transfer. 

 Thus all the energy is directed into the gap in water, where 

 it can be utilized, rather than into the air where its effective- 

 ness is lost and where it is hazardous to the eardrums of the 

 personnel in the area. The vacuum switch used (fig. 4) has 

 a rating of 30 kv at 600 amperes rms, and has been used 

 successfully to carry peak pulse currents of 220, 000 amperes. 



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