Analysis of the Self-Sustaining Processes in a Hollow Cathode using Optical Emission Spectroscopy *

Hollow cathodes are used in just about every plasma process, endemic throughout our lives. The electron emission of the cathode reaches a self-sustaining mode, ion bombardment of the low work-function materials. This research proposes the theory that the self-sustaining mode is a complex result of a mix of ionization states (singly, doubly, ...), the thermodynamic state, the electron energy distribution, and electron production. Electron production in the plasma results from thermionic and secondary emitted electrons. Most computational models assume that only singly ionized particles are present (no plasma is composed of only singly ionized particles). The temperatures in the neutral gas and plasma are often assumed to be the same, but evidence suggest these temperatures are very different. Direct evidence from inside of the hollow cathode is needed to better describe the plasma physical phenomena (ion production, ion-surface impact, electron production at the surface)  Producing accurate measurements of plasma composition, individual species’ temperatures, ionization states, and surface temperatures in a relatively confined space without influencing the plasma directly is the challenge. To produce this evidence, the focus is on developing and using optical emission (non-intrusive) measurement technics  in combination with the branching fraction theory to quantify individual species properties. The final results will then be injected into plasma kinetic formulas and the collisional radiative model (CRM) to further explain the actual physical processes. The results of this research will allow reduced loss designs in the energy production processes, increase communications bandwidth, and reduce energy consumption in manufacturing plating processes.