764 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



The transition from low current to high current in a gas diode can thus 

 be thought of as the process of introducing a space charge of positive ions 

 in the region near the cathode. This is done by raising the voltage tem- 

 porarily above the breakdown value. To switch back to the low current, 

 it is necessary to decrease the multiplication factor M below unity by 

 temporarily lowering the voltage and allowing the ions and excited atoms 

 to diffuse out of the cathode and anode region. Both the turn-on and the 

 turn-off processes impose time restrictions on the switching character- 

 istics. 



The multiplication factor M of (9) applies to an average process. 

 Thus, even though M is greater than unity, it is possible that the ioniza- 

 tion and excitation produced in the gas by any individual electron msLj 

 not release a new electron at the cathode. It is therefore necessary on 

 the average to wait for more than the time between initiating electrons 

 before the discharge starts to build up. 



The average statistical delay is then equal to the average time between 

 successful starting events. If .Vo photoelectrons per second are emitted 

 from the cathode and W is the fraction of these which successfully initiate 

 a discharge, the average statistical delay is^ 



The fraction W -would be expected to increase with an increase in the 

 multiplication factor M and hence with the overvoltage above break- 

 down. It has been shown theoretically and experimentally^ that this 

 is the case. For voltages only slightly in excess of breakdown, i.e., small 

 overvoltages, Fov , the expression for average statistical delay can be 

 approximated by 



^AV ^ ^ (14) 



' ov 



In practical tubes with overvoltages of 10 volts, the average statistical 

 delay may be of the order of milliseconds with radioactive sources of 

 ionization. Short delays of the order of microseconds are obtained by 

 providing an auxiliary "keep-alive" discharge to a separate electrode or 

 by illumination that provides a photoelectric current in the range of 

 10~^^ amperes, 



A formative delay in breakdo^\Ti also occurs because time is required 

 for current to build up to the final value. This time is equal to the 

 product of the number of multiplication cycles and the time per cj'cle. 

 The number of multiplication cycles recjuired is reduced as multiplica- 



