The Self-Sustaining Processes in a Hollow Cathode to Produce a Steady Electron Emission
Hollow cathodes are used in just about every plasma process, endemic throughout our lives (i.e., manufacturing, communications, health care, energy). Performance improvements of the hollow cathode seem to have plateaued, though. The two major characteristics are that 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 different electron production in the plasma with a focus on thermionic and secondary emitted electrons. Most computational models assume that only singly ionized particles are present, but in practice no plasma is composed of only singly ionized particles. The temperatures in the quasi-neutral gas and plasma are often assumed to be the same, but evidence suggest these temperatures are very different. Evidence is needed to better describe the plasma physical phenomena (ion production, ion-surface impact, electron production at the surface) inside of the hollow cathode are the accurate measurements of plasma composition, individual species' temperatures, ionization states, and surface temperatures. Thermocouples can provide some information but not directly in the plasma or on the surface of the insert without changing the operation of the cathode. Langmuir probes have been used to query plasma properties, but only provide global values and are inherently limited by the sheath effect. To produce evidence in support of the proposed theory, this research effort is developed and used non-intrusive optical emission measurement techniques to quantify individual species properties and surface temperatures. This research was then be injected into electron energy distribution model combined with the emission model and collisional radiative model (CRM) to explain the actual physical processes. The results of this research showed distinct thermionic, ionization, and secondary (from ion bombardment) electron processes, as well as traces of primary and secondary ions. This informationcan was used to make recommendations to potentially reduce losses in energy production processes, increase communications bandwidth, and reduce energy consumption in manufacturing plating processes.