Effects of particle-induced electron emission on transport properties in electrically-biased plasma sheaths under fusion-relevant conditions

Abstract

A rigorous implementation of energy-dependent ion- and electron-induced electron emission in a continuum-kinetic framework is used to reveal their effects on the scaling of plasma properties in the sheath with an applied bias potential to the walls. The approach comes with a novel methodology for modeling particle-induced electron emission (PIEE) that includes 1) improved fitting functions for the yield and spectra, 2) the use of SRIM and a summation of the Lindhard formula and a modified Bethe formula for obtaining accurate stopping powers, and 3) a binding energy correction to the PIEE spectra and ion-induced yield based on density functional theory (DFT) calculations. The emission models are implemented as a fully energy dependent and dynamic boundary condition. For this investigation, tungsten and graphite walls are studied for their relevance in magnetic fusion experiments. Equations are derived from fluid theory that predict the relative importance of ion- and electron-induced emission on the structure of the sheath. The simulations provide evidence for the theoretical predictions, showing that a transition from a classical to space-charge limited (SCL) sheath depends primarily on electron-induced emission. Furthermore, claims in previous literature of increased heat and particle loads to the walls due to electron emission are supported, however differing mechanisms for the increase are observed. The increased thermal and particle fluxes due to PIEE are primarily driven by collisional transfer of energy from emitted electrons in the presheath. Finally, quantitative predictions for the device modeled in this study coincide with previous modeling efforts and experimental measurements.