The Electrodynamic Mechanism of Collisionless Multicomponent Plasma Expansion in Vacuum Discharges: From Estimates to Kinetic Theory

Vasily Kozhevnikov,Aleksandr Kokovin,Natalia Semeniuk,Andrey Kozyrev

doi:10.3390/en14227608

Abstract

This paper is devoted to the study of collisionless multicomponent plasma expansion in vacuum discharges. Based on the fundamental principles of physical kinetics formulated for vacuum discharge plasma, an answer is given to the following question: What is the main mechanism of cathode plasma transport from cathode to anode, which ensures non-thermal metallic positive ion movement? Theoretical modeling is provided based on the Vlasov–Poisson system of equations for a current flow in a planar vacuum discharge gap. It was shown that the non-thermal plasma expansion is of a purely electrodynamic nature, caused by the formation of a “potential hump” in the interelectrode space and its subsequent movement under certain conditions consistent with plasma electrodynamic transportation. The presented results reveal two cases of the described phenomenon: (1) the dynamics of single-component cathode plasma and (2) multicomponent plasma (consisting of multiple charged ions) expansion.

Highlights

Almost a hundred years have passed since Tanberg discovered in his experimental paper [1] that in a vacuum arc discharge, the cathode spot is the source of the plasma jet containing heavy metal ions
The case corresponds to quasi-neutral cathode plasma consisting of electrons and a fraction of single- and doublecharged ions
According to the proposed theoretical model based on the fundamental principles of physical kinetics, an explanation of the phenomenon of “anomalous” ion transport of cathode plasma in vacuum diodes was obtained

Summary

Introduction

Almost a hundred years have passed since Tanberg discovered in his experimental paper [1] that in a vacuum arc discharge, the cathode spot is the source of the plasma jet containing heavy metal ions. Numerous studies of many cathode materials and various arc lifetimes have shown that the jet contains a mixture of ions of the cathode material with a predominant fraction of one-, two-, and often three-fold charged ions. Among the key experimental facts established in multiple studies of vacuum discharge, an important role is devoted to the existence of directed ion flows moving toward the anode. Their energies exceed the typical energy value related to the vacuum discharge combustion voltage. It was shown that typical ion motion energy is equal to tens and even hundreds of electron volts, while “thermal” energies lie below 3–5 eV [4]

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Conclusion