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

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° 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°. 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.

References in corpus (2)

• Spatially resolved simulation of a radio frequency driven micro atmospheric pressure plasma jet and its effluent
• Fine-sorting One-dimensional Particle-In-Cell Algorithm with Monte-Carlo Collisions on a Graphics Processing Unit

Cited by in corpus (7)

• The effect of the driving frequency on the confinement of beam electrons and plasma density in low pressure capacitive discharges
• Reproducibility of `COST Reference Microplasma Jets'
• Energy efficiency of voltage waveform tailoring for the generation of excited species in RF plasma jets operated in He/N${}_2$ mixtures
• A New Hybrid Scheme for Simulations of Highly Collisional RF-Driven Plasmas
• Electron heating mode transitions in radio-frequency driven micro atmospheric pressure plasma jets in He/O${}_2$: A fluid dynamics approach
• Electron power absorption in micro atmospheric pressure plasma jets driven by tailored voltage waveforms in He/N${}_2$
• Capacitively-coupled rf discharge with a large amount of microparticles: spatiotemporal emission pattern and microparticle arrangement