Resolving the mystery of electron perpendicular temperature spike in the plasma sheath

TL;DR

This paper investigates the cause of the electron perpendicular temperature spike in plasma sheaths, attributing it to negative heat flux gradients driven by pitch-angle scattering in various collisional regimes.

Contribution

It identifies the physical mechanism behind the $T_{eot}$ spike as a negative heat flux gradient caused by pitch-angle scattering, unifying different collisional regimes.

Findings

The $T_{eot}$ spike results from a negative heat flux gradient.

Pitch-angle scattering induces non-zero heat flux in the sheath.

The phenomenon occurs across different plasma collisionality regimes.

Abstract

A large family of plasmas has collisional mean-free-path much longer than the non-neutral sheath width, which scales with the plasma Debye length. The plasmas, particularly the electrons, assume strong temperature anisotropy in the sheath. The temperature in the sheath flow direction ($T_{e\parallel}$) is lower and drops towards the wall as a result of the decompressional cooling by the accelerating sheath flow. The electron temperature in the transverse direction of the flow field ($T_{e\perp}$) not only is higher but also spikes up in the sheath. This abnormal behavior of $T_{e\perp}$ spike is found to be the result of a negative gradient of the parallel heat flux of transverse degrees of freedom ($q_{es}$) in the sheath. The non-zero heat flux $q_{es}$ is induced by pitch-angle scattering of electrons via either their interaction with self-excited electromagnetic waves in a nearly collisionless plasma or Coulomb collision in a collisional plasma, or both in the intermediate regime of plasma collisionality.