Thermionic arc breakdown

A model for thermionically induced breakdown is developed which predicts the voltage levels experienced in low-voltage discharges. The scheme requires the establishment of a continuum of electron emission across the primary discharge gap but the effective discharge initiation length must be small. The source of electrons must initially be independent of the discharge itself, being activated prior to breakdown. Electron emission therefore is achieved by auxiliary means which may include field and even photo emission in addition to thermionic emission. Before breakdown, these emitted electrons remain in the space-charge region adjacent to the cathode surface. Breakdown criteria and a means for estimating the reduction in breakdown voltage are derived in terms of conventional Townsend ionization mechanisms. Specific relevant processes are multistep or cumulative ionization and partial neutralization of the cathode space charge by positive ions. Because the cathode space-charge region is replete with emitted electrons, ions traversing it release considerably more electrons than would be available with the conventional Townsend γ-mechanism at the surface. This equivalently higher γ significantly shifts the Paschen curve toward lower minima. One thermionic assisting device proposed is a tungsten light-bulb-grade filament which shunts the primary discharge gap. Thermionic arc breakdown has been implemented in the starting of arc devices for lamps and in thermionic power converters. A possible application to ionization chambers is also discussed