Torben Hemke

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

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 ??-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 at 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.

Spatial dynamics of helium metastables in sheath or bulk dominated rf micro-plasma jets

Space resolved concentrations of helium He metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. The spatial profile of metastable atoms in the volume between the electrodes was deduced for various electrode gap distances. Density profiles reveal the sheath structure and reflect the plasma excitation distribution, as well as the dominance of the α-mode discharge. Gap width variations show the transition from a normal glow plasma to a pure sheath discharge. In order to analyse and verify the experimentally observed profiles of the metastable atoms, a two-dimensional simulation model was set up. Applying an appropriate He/N2/O2 chemistry model, the correlation between the metastable profiles and the underlying excitation mechanisms was obtained.

Simulations of electromagnetic effects in high-frequency capacitively coupled discharges using the Darwin approximation

The Darwin approximation is investigated for its possible use in simulation of electromagnetic effects in large size, high-frequency capacitively coupled discharges. The approximation is utilized within the framework of two different fluid models which are applied to typical cases showing pronounced standing wave and skin effects. With the first model it is demonstrated that the Darwin approximation is valid for treatment of such effects in the range of parameters under consideration. The second approach, a reduced nonlinear Darwin approximation-based model, shows that the electromagnetic phenomena persist in a more realistic setting. The Darwin approximation offers a simple and efficient way of carrying out electromagnetic simulations as it removes the Courant condition plaguing explicit electromagnetic algorithms and can be implemented as a straightforward modification of electrostatic algorithms. The algorithm described here avoids iterative schemes needed for the divergence cleaning and represents a fast and efficient solver, which can be used in fluid and kinetic models for self-consistent description of technical plasmas exhibiting certain electromagnetic activity.

Spatially resolved simulation of a radio-frequency driven micro-atmospheric pressure plasma jet and its effluent

Radio-frequency driven plasma jets are frequently employed as efficient plasma sources for surface modification and other processes at atmospheric pressure. The radio-frequency driven micro-atmospheric pressure plasma jet (μAPPJ) is a particular variant of that concept whose geometry allows direct optical access. In this work, the characteristics of the μAPPJ operated with a helium–oxygen mixture and its interaction with a helium environment are studied by numerical simulation. The density and temperature of the electrons, as well as the concentration of all reactive species are studied both in the jet itself and in its effluent. It is found that the effluent is essentially free of charge carriers but contains a substantial amount of activated oxygen (O, O3 and O2(1Δ)).The simulation results are verified by comparison with experimental data.

Nonlocal behavior of the excitation rate in highly collisional RF discharges

The present work focuses on the fundamental aspects of atmospheric pressure plasma electropositive discharges operated in the ohmically heated mode, the electron heating and the excitation (ionization) rate. We find that the two do not necessarily have similar profiles and can show peaks at different locations, the ionization rate being much more sensitive to the electric field compared to the sensitivity to the electric field of the electron heating. This suggests an explanation for the discrepancies between the profiles of the power absorbed by electrons and the excitation patterns previously reported in the literature and observed in the present study. The excitation rate profile can then be explained by analyzing overlapping of the electron heating and the electric field profiles. Surprisingly, it has been discovered that the excitation dynamics exhibits nonlocal behavior having maxima spatially separated from the maxima of the electric field and the electron heating rate, a new effect in discharges operated in the mode. The strong electric field in such discharges leads to large displacements of the electron component. This can produce significant charge separation close to the sheath or even in the bulk plasma because electrons are not able to follow the electric field adiabatically and maintain quasineutrality owing to the high collisionality. In particular, this leads to a significant distortion of the sheath structure and increase in the electric field there.

Ionization wave propagation on a micro cavity plasma array

The simulation was performed using the computer modeling platform nonPDPSIM, described in detail in Refs. 10–12 and briefly discussed here. Poisson’s equation for the electrostatic potential is self-consistently coupled with driftdiffusion equations for the transport of charged species and the surface charge balance equation. The set of equations is simultaneously integrated in time using an implicit Newton iteration technique. This integration step is followed by an implicit update of the electron temperature by solving the electron energy equation. To capture the non-Maxwellian behavior of the electrons, the electron transport coefficients and rate coefficients are obtained by solving the zerodimensional Boltzmann’s equation for the electron energy distribution. A Monte Carlo simulation is used to track the trajectories of sheath accelerated secondary electrons. The transport of photons is treated by means of a Green’s function propagator. The discharge is sustained in argon at atmospheric pressure. The species in the model are electrons, Ar(3s), Ar(4s), Ar(4p), Ar þ ,A r �, and Ar þ . The photon transport we tracked in the model is dimer radiation from Ar � .I n

Ignition of a Microcavity Plasma Array

Microcavity plasma arrays are regular arrays of inverse pyramidal cavities created on positively doped silicon wafers. Each cavity acts as a microscopic dielectric barrier discharge. It has an opening of 50 μm × 50 μm and a depth of 45 μm. The separation of the cavities is 50 μm. Operated at atmospheric pressure in argon and excited with a 100-kHz RF voltage, each cavity develops a localized microplasma. Experiments show a strong interaction of the individual cavities, leading, for example, to the propagation of ionization waves along the array surface. This paper studies the ignition of a microcavity plasma array by means of a numerical simulation. The propagation of an ionization wave is observed. Its propagation speed matches experimental findings.

On the importance of N4+ ions in the chemistry of a He-N microjet discharge

Summary form only given. The microdischarges under atmospheric pressure do not require expensive vacuum equipment and therefore are potentially advantageous for industrial and biological applications. The experimental diagnostics options are however rather limited due to the small size of the microdischarges and numerical modeling is thus very useful for understanding of the processes taking place there. One of the examples frequently treated in the literature is a He-N microjet discharge driven by an RF voltage source. Most of the treatments in the literature do not include the N4+ cluster ions in their chemistry sets. We demonstrate, by modeling a He-N microjet discharge with a hybrid code which treats electrons kinetically and ions using a fluid description, that N4+ ions tend to be the dominant ion species in such a discharge even for small fractions of He. We also discuss several additional interesting features of the discharge.

Kinetic simulations of a large-sized multifrequency CCP-based sputtering source with a PIC/MCC darwin code

Summary form only given. A novel concept of a sputtering source based on a CCP multifrequency large-sized discharge is currently under experimental investigation [1]. The physics of such a discharge is quite complex and includes phenomena taking place on several time and spatial scales. In particular, because of the size and the high frequency harmonics in the driving voltage of such a discharge, the electromagnetic effects may play a significant role. Moreover, use of the electrical asymmetry effect (EAE) to create a self-consistent bias complicates the problem even more. In the present work we report results of our studying such a discharge with a recently developed self-consistent kinetic 2d3c PIC/MCC GPU-parallelized code which uses Darwin approximation [2] for description of the electromagnetic field components. The simulations are made in a geometry close to that of the sputtering source used in the experiments. We discuss interesting features of the discharges arising in the main and the side chambers and compare the simulation results and the experimental data.

Numerical simulation of a coaxial microplasma jet at atmospheric pressure

Summary form only given. Microplasmas at or around atmospheric pressure gained rising attention recently. Their technological benefits and application-oriented flexibility allows microplasmas to explore new fields of plasma technology, e.g. biomedical applications. One particular type of microplasma sources that shows a variety of interesting physics and applications is the so called cold microplasma jet.