Hollow Cathode Ignition and Life Model

The Hollow Cathode Assembly is a thermionic source of electrons critical to the operation of the L-3 ETI Xenon Ion Propulsion System (XIPS) thruster. There are two cathode subsystems on a XIPS thruster: the discharge cathode provides the current for the main discharge and the neutralizer cathode produces an electron stream to prevent the spacecraft from charging. There are two phases to the operation of the cathode: 1) a pre-heat phase during which an external heater prepares the cathode for ignition and 2) a self-heating phase that follows ignition. We will describe a detailed physical model of the cathode. Use of the model to characterize the expected life of the device was validated using published NASA data and internal L-3 ETI life test data. Nomenclature P0 = barium pressure in tungsten pore at the burn front Ps = barium pressure in tungsten pore at the emission surface Ω(T) = time required to completely consume barium at temperature, T G = surface deposition of barium integrated over time θ = barium surface coverage φ = work function Γ = depth of the work function minimum j = current density i = emitted current I