A Penning gas mixture consists of a rare gas containing impurity atoms possibly at
very low concentrations. The impurity atoms have an
ionization potential
which is lower than or equal to the
metastable potential
(
)
of the parent noble gas. The Penning effect in a Penning gas mixture is the
ionization by charge transfer (charge exchange) during collision between a
metastable atom and a neutral atom which decreases the average energy to form an
ion pair, e.g.
In a glow discharge, this results in an increase of the the
ionizationcoefficient (Townsend first
coefficient), a decrease in breakdown potential and a lowering of the
cathode fall potential. The magnetic Penning effect describes the increase of the
ionizationprobability of gas in a
low pressure electrical discharge resulting from the helical (spiral) movement of electrons in a magnetic field placed
normal to the
anode-
cathode electrical field.
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