Effects of oxidation and impurities in lithium surfaces on the emitting wall plasma sheath

TL;DR

This study uses simulations to analyze how oxidation and impurities on lithium surfaces alter plasma sheath structures and electron emission, impacting wall erosion and energy transfer in fusion devices.

Contribution

It introduces a data-driven emission model that captures the effects of oxidation on sheath structure and electron emission in lithium-coated walls.

Findings

Oxidation causes a transition from monotonic to nonmonotonic sheath structures.

Increased secondary electron emission leads to higher electron flux and energy transfer.

Oxidized lithium surfaces exhibit higher re-emission rates due to backscattering.

Abstract

Use of lithium as a surface coating in fusion devices improves plasma performance, but the change in wall properties affects the secondary electron emission properties of the material. Lithium oxidizes easily, which drives the emission yield well above unity. We present here simulations demonstrating the change in sheath structure from monotonic to the nonmonotonic space-charge limited sheath using an energy-dependent data-driven emission model which self-consistently captures both secondary emission and backscattering populations. Increased secondary electron emission from the material has ramifications for the degradation and erosion of the wall. Results shows that the oxidation leads to an increased electron flux into the wall, and a reduced ion flux. The net transfer of energy to the surface is significantly greater for the oxidized case than for the pure lithium case. High reflection rates of low-energy backscattered particles leads to a high re-emission rate at the wall.