Penning gas mixture

A Penning gas mixture consists of a rare gas containing impurity atoms possibly at very low concentrations. The impurity atoms have an ionization potentialV ion which is lower than or equal to the metastable potential (V meta) 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.
Orange Book, p. 148