Plasma-Material Interactions for Electric Propulsion and Plasma-Facing Components

Book excerpt: Quantifying lifetime and performance of electric propulsion devices is inherently challenging as device operations last on the order of thousands of hours. Physics-based approaches to capture the key mechanisms of particle or plasma-material interaction with components are required. The first limiting factor for electrospray thruster is propellant flux to and interaction with the grids, resulting in saturation and electrical failures. The process is primarily driven by impinging mass flux, as demonstrated by the first life model developed considering mass flux for evaluating lifetime of capillary droplet-mode electrospray thrusters. Mass flux was classically assumed gaussian in shape due to collisions and scattering processes, but novel measurements demonstrate that mass flux is super-gaussian in nature. The measurements provide novel insight on electrospray plume structure and how operating conditions affect their shape. The super-gaussian nature of the profile indicates the need to consider mass flux in addition to current for thruster performance and lifetime evaluation. Plasma material interactions are also a challenge for lifetime and performance of components for electric propulsion and fusion applications. Ion bombardment and sputtering degrades boundary materials and contaminants contaminates the plasma, reducing performance, limiting device lifetime, and increases component replacement costs. Plasma-infused volumetrically complex foams have been shown to persistently reduce sputtering-yield. In this work, experimental measurements of a foam under plasma exposure demonstrate the ability of the foam to partially-infused with plasma and provide insight on the plasma-infusion process dependence on plasma properties. PPI is shown to be the key design parameter to create VCMs for specific plasma properties and infusion behavior. Additionally, a novel sputtering model is developed to accurately describe the sputtering distributions and trapping mechanisms of VCMs of different materials and energies. The model demonstrates a key milestone in the development of VCMs to reduce sputtering yield up to 70%. The aspect ratio is shown to be the primary driver for sputtering behavior in the plasma-facing region and can be used tailor a VCM for a specific application independent of PPI, which can be tailored for specific plasma properties. Additionally, the role of azimuthal instabilities in ExB systems on the PMI process is characterized. Characterization of the mode provides insight on the mode dependence on plasma properties, with implications for Hall thrusters, low-temperature plasma devices, and plasma-material interaction characterization of volumetrically-complex materials. Azimuthal instabilities must be accounted for as they are shown to be non-negligible in certain scenarios and significantly impact system dynamics via azimuthal velocity components.