FLEXIBLE REPEATER DESIGN 



89 



from the cable allows the tube to deionize and recover in the event of 

 false triggermg by transients. The cathode is a coil of tungsten wire coated 

 with a mixture of barium and strontium oxide. A cold cathode glow dis- 

 charge forms when the tube is first broken down. This discharge has a 

 sustaining voltage of the order of 70 volts. The glow discharge initially 

 covers the entire cathode area. Local heating occurs and some parts of 

 the oxide coating begin to emit electrons thermionically. This local emis- 

 sion causes increased current density and further increases the local 

 heating. The discharge thus concentrates to a thermionic arc covering 

 only a portion of the coil. The sustaining voltage is then of the order of 

 10 volts. 



Mechanically the tube was designed to minimize the possibility of a 

 short circuit resulting from structural failure of tube parts. Fig. 16 shows 

 the construction of the tube. The glass envelope and stem structure which 

 had previously been developed for the hot cathode repeater tubes were 

 used as a starting point for the design. The anode is a circular disk of 

 nickel attached to two of the stem lead wires. To provide shock resistance 

 the supporting stem leads are crossed and welded in the center. To pro- 

 tect against weld failure, a nickel sleeve is used at each end of the cathode 



Fig. 16 — The power by-pass gas tube. 



