Cold-cathode discharges and breakdown in argon: surface and gas phase production of secondary electrons

We review the data and models describing the production of the electrons, termed secondary electrons, that initiate the secondary and subsequent feedback avalanches required for the growth of current during breakdown and for the maintenance of low-current, cold-cathode discharges in argon. First we correlate measurements of the production of secondary electrons at metallic cathodes, i.e. the yields of electrons induced by Ar⁺ ions, fast Ar atoms, metastable atoms and vuv photons. The yields of electrons per ion, fast atom and photon vary greatly with particle energy and surface condition. Then models of electron, ion, fast atom, excited atom and photon transport and kinetics are fitted to electrical-breakdown and low-current, discharge-maintenance data to determine the contributions of various cathode-directed species to the secondary electron production. Our model explains measured breakdown and low-current discharge voltages for Ar over a very wide range of electric field to gas density ratios E/n, i.e. 15 Td to 100 kTd. We review corrections for nonequilibrium electron motion near the cathode that apply to our local-field model of these discharges. Analytic expressions for the cross sections and reaction coefficients used by this and related models are summarized.