Thermionic Cooling of the Target Plasma to a Sub-eV Temperature

TL;DR

This paper reveals that sheath inversion in collisional plasmas can cool the target plasma to near the emitted electron temperature, significantly impacting plasma-surface interactions and enabling new tokamak divertor strategies.

Contribution

It demonstrates that sheath inversion causes the target plasma temperature to match emitted electron temperature, challenging previous models and suggesting novel plasma control methods.

Findings

Sheath inversion leads to extreme cooling of the target plasma.

Target plasma temperature can equal emitted electron temperature despite hot upstream plasma.

Potential application in inducing detachment in tokamaks.

Abstract

Contemporary models of bounded plasmas assume that the target plasma electron temperature far exceeds the temperature of the cold electrons emitted from the target, T_emit. We show that when the sheath facing a collisional plasma becomes inverted, the target plasma electron temperature has to equal T_emit even if the upstream plasma is hotter by orders of magnitude. This extreme cooling effect can alter the plasma properties and the heat transmission to thermionically emitting surfaces in many applications. It also opens a possibility of using thermionic divertor plates to induce detachment in tokamaks.