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1. HIGH VOLTAGE
ENGINEERING
 TOWNSEND’S
IONIZATION
COEFFICIENT
IN SIMPLE
LAUNGUAGE
LEC # 05
BREAKDOWN
IN GASES

2.  When one electron collides with a neutral particle, a positive ion and an
electron are formed. This is called an ionizing collision.
 Assume n0 is electrons emitted from cathode. Let α be the average number
of ionizing collisions made by an electron per cm travel in the direction of the
field where it depends on gas pressure (P) and E/P, and is called the
Townsend’s first ionization coefficient or primary ionization coefficient.
 x is a distance from cathode and nx is number of electrons.When these nx
electrons travel a further distance of dx they give rise to (α nx dx)Electrons.
TOWNSEND’S PRIMARY IONIZATION COEFFICIENT

3. TOWNSEND’S SECONDARY IONIZATION COEFFICIENT
 The electrons produced by these processes are called secondary
electrons. The secondary ionization coefficient is γ.
 Once Townsend’s primary ionization is initiated secondary ionization
processes follow resulting in the final breakdown of gases.
 These processes are :
 Ionization due to positive ions.
 Photo ionization
 Liberation of electrons from cathode due to positive ions
bombardment.
 The metastable particles may diffuse back causing electron emission

4. TOWNSEND’S SECONDARY IONIZATION COEFFICIENT
 The net number of secondary electrons
produced per incident positive ion,
photon, metastable particle, and the total
value of γ is the sum of the individual
coefficients due to the three different
processes, i.e., γ= γ1+ γ2 + γ3.
 γ is called the Townsend's secondary
ionization coefficient and is a function of
the gas pressure p and E/p.

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