Lithium-Fed Hollow Cathode Theory

A theory for the single-channel hollow cathode (SCHC) that includes the relevant physical processes and predicts the operating parameters is presented. A SCHC consists of a cylindrical channel approximately ten diameters in length through which a working gas (taken as lithium vapor here) flows and exits as a plasma. A high current (1-100 A) is conducted into the channel by the plasma and to the cathode walls via field-enhanced thermionic emission and the plasma ions. The cathode voltage is determined from an energy balance that accounts for surface processes. An energy balance in the plasma volume determines the electron temperature. The SCHC theory includes, for the first time, a model of non-equilibrium excitation and ionization via thermionic and thermal electron collisions that utilizes a rate balance of the important excitation states of the lithium neutrals. The model predicts important operating parameters including the cathode voltage, temperature profile, and ionization fraction as a function of current, lithium flow rate, and channel diameter. The phenomenon of the minimum voltage as a function of current is captured by the model. The model gives insight into the primary loss mechanisms. It was found that thermal radiation dominates at low current, thermionic cooling dominates at moderate current, and convection of thermal electron energy dominates at high current.