Plasma sheath in the presences of non-Maxwellian energetic electrons and secondary emission electrons

Abstract

The formation of a sheath in front of a carbon or tungsten material plane immersed in a plasma containing non-Maxwellian energetic electrons and secondary emission electrons is studied using a 1D model. In the model, energetic electrons are described by the electron energy distribution function (EEDF) and secondary electron emission (SEE) is produced by the electrons impinging on the wall. It is found that SEE coefficient depends on not only the sheath potential but also the EEDF profile of energetic electrons when a non-Maxwellian energetic electron component is present. The energetic electrons and associated secondary emission electrons can strongly modify ion velocity at sheath edge, floating potential and I-V probe characteristic. Due to the interdependence between SEE coefficient originating from the impact of non-Maxwellian energetic electrons on the wall and the sheath potential, with the increase in the energy of energetic electrons, a sudden jump phenomenon can be found in the profiles of SEE coefficient and other quantities such as floating potential and ion velocity at the sheath edge for tungsten wall, while for carbon wall they are the continuous variation. To begin with, the energetic electron component does not dominate the sheath, and I-V probe characteristic depends on both the EEDF profile of energetic electrons and material properties. Once the energetic electron component dominates the sheath, the analysis of I-V probe characteristic will yield the energy of energetic electrons.

A tutorial on radio frequency sheath physics for magnetically confined fusion devices

“…Before closing this short section on static sheaths, it should be noted that many more sophisticated models have been developed and studied to take into account effects such as secondary electron emission (Stangeby 2000;Campanell & Umansky 2017), non-Maxwellian electron distributions (Ou et al 2016), collisions (Tang & Guo 2015), ionization and kinetic ion effects (Khaziev & Curreli 2015) and E × B and diamagnetic drifts (Cohen & Ryutov 1999;Stangeby 2000). These are but a few of the many hundreds of papers in the literature on these and related topics.…”

Heat flow through a plasma sheath in the presence of secondary electron emission from plasma‐wall interaction

“…If the electrons obey a non-Maxwellian electron energy distribution, eff depends on both the sheath potential and their energy when they impinge the material surface. [7] . However, for a Maxwellian distribution of the electrons, it does not depend on sheath potential, but is usually a function of the electron temperature.…”

“…However, for a Maxwellian distribution of the electrons, it does not depend on sheath potential, but is usually a function of the electron temperature. [2,7,28] By using the Sternglass formula e ( ) = m m exp(2 − 2 √ ∕ m ), [29] where m is the maximum of e , and m is the characteristic energy corresponding to m , eff can be expressed as, [28]…”

“…The characteristics of a plasma sheath located between a plasma and a material surface play an important role in the plasma heat flow to the material surface during plasma-wall interaction. Several key factors influence the sheath structure, including electron emission, [1][2][3][4][5][6][7][8][9][10][11] magnetic field, [11][12][13][14][15] ion temperature, [13][14][15] and collision. [15][16][17] The most important one is the electron emission from plasma-wall interaction.…”

Energetic Ion Effects on the Ion Saturation Current

“…[3,4] Many studies have been carried out for the sheath formation in a plasma containing high-energy electron beams. [5,6] However, the energetic ion effects on the ion saturation current have received only slight attention so far. The presence of energetic ions may make a significant contribution to the ion saturation current, indicating that the classical probe theory which excludes the energetic ion effect will lead to inaccurate measurements of the plasma temperature and floating potential.…”