Theory and simulation of anode spots in low pressure plasmas

TL;DR

This paper combines 2D particle-in-cell simulations and a theoretical model to analyze the formation and properties of anode spots in low-pressure plasmas, revealing the role of ionization and potential wells.

Contribution

It presents the first 2D PIC simulations of anode spot formation and develops a theoretical model predicting their size, potential drop, and sheath structure.

Findings

Ionization causes ion-rich layers near electrodes.

Anode spots form when a quasineutral region exceeds Langmuir conditions.

Model predictions align with simulations and experiments.

Abstract

When electrodes are biased above the plasma potential, electrons accelerated through the associated electron sheath can dramatically increase the ionization rate of neutrals near the electrode surface. It has previously been observed that if the ionization rate is great enough, a double layer separates a luminous high-potential plasma attached to the electrode surface (called an anode spot or fireball) from the bulk plasma. Here, results of the first 2D particle-in-cell simulations of anode spot formation are presented along with a theoretical model describing the formation process. It is found that ionization leads to the buildup of an ion-rich layer adjacent to the electrode, forming a narrow potential well near the electrode surface that traps electrons born from ionization. Anode spot onset occurs when a quasineutral region is established in the potential well and the density in this region becomes large enough to violate the steady-state Langmuir condition, which is a balance between electron and ion fluxes across the double layer. A model for steady-state properties of the anode spot is also presented, which predicts values for the anode spot size, double layer potential drop, and form of the sheath at the electrode by considering particle, power, and current balance. These predictions are found to be consistent with the presented simulation and previous experiments.