Environmental Engineering Reference
In-Depth Information
quantity for the ionization process by electron impact in a gas in an external electric
field is the first Townsend coefficient
α
,whichisdefinedas
α D v 0
k ion
if we assume that the ionization process proceeds near the ionization threshold.
The first Townsend coefficient is the reciprocal value of the electron mean free
path with respect to ionization. Figures 3.8 and 3.9 show the measured values of
the first Townsend coefficient for helium and argon [32].
The first Townsend coefficient is a convenient characteristic for description of the
principal properties of gas discharge as the passage of an electric current through
a gas under the action of an external electric field. Indeed, how to produce gas
discharge and its basic forms were worked out in the nineteenth century, whereas
the understanding of gas discharge as a self-maintaining phenomenon caused by
Figure 3.8 The first Townsend coefficient in helium [32].
Figure 3.9 The first Townsend coefficient in argon [32].
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