Ionization by bulk heating of electrons in capacitive radio frequency atmospheric pressure microplasmas

TL;DR

This study investigates electron heating and ionization in RF atmospheric pressure microplasmas using simulations and models, revealing a dominant ohmic heating mechanism driven by high collision frequencies, with ionization phase depending on plasma density and voltage amplitude.

Contribution

It introduces a semi-analytical model explaining electron heating and ionization dynamics in atmospheric microplasmas, highlighting the Omega-mode and phase behavior related to plasma density and voltage.

Findings

Ionization mainly caused by ohmic heating in the plasma bulk.

Phase of maximum ionization depends on plasma density and voltage amplitude.

Analogies found with low-pressure electronegative discharges in Drift-Ambipolar mode.

Abstract

Electron heating and ionization dynamics in capacitively coupled radio frequency (RF) atmospheric pressure microplasmas operated in helium are investigated by Particle in Cell simulations and semi-analytical modeling. A strong heating of electrons and ionization in the plasma bulk due to high bulk electric fields are observed at distinct times within the RF period. Based on the model the electric field is identified to be a drift field caused by a low electrical conductivity due to the high electron-neutral collision frequency at atmospheric pressure. Thus, the ionization is mainly caused by ohmic heating in this "Omega-mode". The phase of strongest bulk electric field and ionization is affected by the driving voltage amplitude. At high amplitudes, the plasma density is high, so that the sheath impedance is comparable to the bulk resistance. Thus, voltage and current are about 45{\deg} out of phase and maximum ionization is observed during sheath expansion with local maxima at the sheath edges. At low driving voltages, the plasma density is low and the discharge becomes more resistive resulting in a smaller phase shift of about 4{\deg}. Thus, maximum ionization occurs later within the RF period with a maximum in the discharge center. Significant analogies to electronegative low pressure macroscopic discharges operated in the Drift-Ambipolar mode are found, where similar mechanisms induced by a high electronegativity instead of a high collision frequency have been identified.