Influence of a readily ionized impurity on the ignition voltage of a bulk discharge in molecular gases

An experimental study of how the initial ratio of the electric field strength in the discharge gap to the gas pressure E(0)/p depends on the pressure of N2, CO2, and He gases and their mixtures is presented along with a study of how addition of a readily ionized impurity influences the pressure dependence of E(0)/p. It was found that when no He is present, the ignition threshold for formation of a self-sustaining bulk discharge is a minimum for CO2:N2 = 1:1. An increase in the partial CO2 pressure causes E(0)/p to increase. An unexpected finding is that E(0)/p drops when the N2 content is increased above 50 percent, and the discharge parameters are less reproducible. Addition of TPA does not greatly alter the discharge ignition voltage for mixtures which do not contain N2. A reduced discharge ignition voltage exists in mixtures containing N2 and TPA, apparently due to Penning ionization of TPA involving the metastable electronic states of N2.