Velocity Distribution Functions in Electron-beam-assisted Plasmas

Velocity distribution functions (VDF) are essential to understand plasma behavior at different scales. Fluid moments are obtained from the measured distribution function including the population density, bulk flow, and temperature of the interrogated species. Additionally, VDFs provide insight into several plasma phenomena including wave-particle interactions, instabilities, and sheath dynamics. In this work, we use nonperturbative and spatially-localized laser-induced fluorescence (LIF) techniques to resolve VDFs in electron-beam-assisted plasmas. Unlike probe-based diagnostics such as Langmuir probes or Retarding Field Energy Analyzers (RFEAs), LIF is a non-invasive diagnostic, with excellent spatial localization and temporal resolution. To date, two-photon absorption laser-induced fluorescence (TALIF) measurement of helium ions have not been realized. Utilizing electron beams is proposed as a method to populate targeted electronic states which are otherwise inaccessible to standard two-photons laser-induced florescence (TALIF) schemes. In addition to that, high-energy electron beams interact with plasma environments which allows us to study the plasma behavior under different regimes. In this work, we present the most recent results from LIF measurements of argon and helium in electron-beam-assisted plasmas.