A new 2D fluid-MC hybrid approach for simulating nonequilibrium atmospheric pressure plasmas: density distribution of atomic oxygen in radio-frequency plasma jets in He/O<sub>2 </sub>mixtures
Plasma Sources Science and Technology, Jan 15, 2024
Plasma Sources Science and Technology, Sep 6, 2019
We investigate the spatially and temporally resolved electron kinetics in a homogeneous electric ... more We investigate the spatially and temporally resolved electron kinetics in a homogeneous electric field in argon gas, in the vicinity of an emitting boundary. This (transient) region, where the electron swarm exhibits non-equilibrium character with energy gain and loss processes taking place at separate positions (in space and time), is monitored experimentally in a scanning drift tube apparatus. Depending on the strength of the reduced electric field we observe the equilibration of the swarm over different length scales, beyond which the energy gain and loss mechanism becomes locally balanced and transport properties become spatially invariant. The evolution of the electron swarm in the experimental apparatus is also described by Monte Carlo simulations, of which the results are in good agreement with the experimental observations, over the domains of the reduced electric field and the gas pressure covered.
Plasma Sources Science and Technology, Apr 29, 2020
The spatio-temporal ionization and excitation dynamics in low-pressure radiofrequency (RF) discha... more The spatio-temporal ionization and excitation dynamics in low-pressure radiofrequency (RF) discharges operated in neon are studied and a detailed comparison of experimental and kinetic simulation results is provided for a wide parameter regime. Phase resolved optical emission spectroscopy (PROES) measurements and 1d3v particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations are performed in a geometrically symmetric capacitively coupled plasma (CCP) reactor at driving frequencies ranging from 3.39 MHz to 13.56 MHz, pressures between 60 Pa and 500 Pa, at a peak-to-peak voltage of 330 V. We examine the applicability of PROES (which provides information about the spatio-temporal distribution of the electron-impact excitation dynamics from the ground state into the Ne 2p 1 state) to probe the discharge operation mode in neon (which is determined by the spatio-temporal distribution of the ionization dynamics). We nd that the spatio-temporal excitation rates measured by PROES are in a good agreement with the excitation rates obtained from the PIC/MCC simulations, for all the discharge conditions studied here. However, the ionization dynamics is found to be signi cantly different from the excitation dynamics under most of the discharge conditions studied here, especially at higher values of the driving frequency and lower values of the pressure, when energetic heavy particle induced secondary electrons (γ-electrons) are more likely to ionize than to excite. PROES does not probe the discharge operation mode under these conditions. At a xed frequency and peak-to-peak voltage, the spatio-temporal distribution of the ionization rate obtained from PIC/MCC simulations shows a transition of the discharge operation mode from the α-mode to the γ-mode by increasing the pressure. However, PROES fails to show this transition. While in the spatio-temporal distribution of the excitation rate obtained from the PROES measurements and the PIC/MCC simulations the α-peak (the intensity maximum at the bulk side of the expanding sheath edge) is dominant and a γ-peak (a maximum near the edge of the fully expanded sheath) becomes visible only at high values of the pressure or at the lowest frequency of 3.39 MHz, a γ-peak is visible in the 5 Author to whom any correspondence should be addressed.
We study binary and the recently discovered process of ternary He-assisted recombination of H + 3... more We study binary and the recently discovered process of ternary He-assisted recombination of H + 3 ions with electrons in a low temperature afterglow plasma. The experiments are carried out over a broad range of pressures and temperatures of an afterglow plasma in a helium buffer gas. Binary and He-assisted ternary recombination are observed and the corresponding recombination rate coefficients are extracted for temperatures from 77 K to 330 K. We describe the observed ternary recombination as a two-step mechanism: First, a rotationally-excited long-lived neutral molecule H * 3 is formed in electron-H + 3 collisions. Second, the H * 3 molecule collides with a helium atom that leads to the formation of a very long-lived Rydberg state with high orbital momentum. We present calculations of the lifetimes of H * 3 and of the ternary recombination rate coefficients for para and ortho-H + 3. The calculations show a large difference between the ternary recombination rate coefficients of ortho and para-H + 3 at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.
Bulletin of the American Physical Society, Oct 14, 2015
The plasma series resonance is a fundamental phenomenon due to the nonlinear interaction between ... more The plasma series resonance is a fundamental phenomenon due to the nonlinear interaction between the plasma bulk and the sheaths of a capacitive discharge. It has been proven to play an important role in the context of electron heating. Furthermore, recent results indicate that the manifestation of harmonics in the plasma current due to the plasma series resonance is responsible for a nonlinear standing wave effect and, consequently, spatial inhomogeneities in the power deposition. It is important to note that the plasma series resonance is a current governed resonance and that it can only be excited in voltage driven systems. Particle-In-Cell simulations however show that also in current driven systems resonances can occur due to the non-linear excitation of harmonics in the conduction and displacement current. In this work, the differences between voltage and current driven capacitive discharges in terms of their nonlinear behavior are investigated. It is found that under certain conditions nonlinear plasma parallel resonances are excited which are able to support the electron heating.
We investigate the electron heating dynamics in electropositive argon and helium capacitively cou... more We investigate the electron heating dynamics in electropositive argon and helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in Cell simulations and by an analytical model. The model allows to calculate the electric field outside the electrode sheaths, space and time resolved within the RF period. Electrons are found to be heated by strong ambipolar electric fields outside the sheath during the phase of sheath expansion in addition to classical sheath expansion heating. By tracing individual electrons we also show that ionization is primarily caused by electrons that collide with the expanding sheath edge multiple times during one phase of sheath expansion due to backscattering towards the sheath by collisions. A synergistic combination of these different heating events during one phase of sheath expansion is required to accelerate an electron to energies above the threshold for ionization. The ambipolar electric field outside the sheath is found to be time modulated due to a time modulation of the electron mean energy caused by the presence of sheath expansion heating only during one half of the RF period at a given electrode. This time modulation results in more electron heating than cooling inside the region of high electric field outside the sheath on time average. If an electric field reversal is present during sheath collapse, this time modulation and, thus, the asymmetry between the phases of sheath expansion and collapse will be enhanced. We propose that the ambipolar electron heating should be included in models describing electron heating in capacitive RF plasmas.
Bulletin of the American Physical Society, Nov 6, 2018
Microscopic atmospheric pressure plasma jets are important tools for biomedical applications and ... more Microscopic atmospheric pressure plasma jets are important tools for biomedical applications and surface modifications. They are typically operated at a single driving frequency with limited control of the electron power absorption dynamics and the Electron Energy Distribution Function (EEDF). For such applications the generation of reactive species, e.g. reactive oxygen and nitrogen species, at low temperatures plays a key role. Based on experiments and kinetic Particle-in-Cell/Monte Carlo simulations, we demonstrate that Voltage Waveform Tailoring (VWT) allows to control the spatio-temporal excitation/ionization dynamics and the EEDF as the basis to optimize the generation of selected reactive particle species.
Bulletin of the American Physical Society, Oct 29, 2014
The self-excitation of plasma series resonance (PSR) oscillations is a dominant feature in the cu... more The self-excitation of plasma series resonance (PSR) oscillations is a dominant feature in the current of asymmetric capacitively coupled radio-frequency discharges. The asymmetry can be caused by an asymmetry of the chamber geometry and/or that of the applied voltage waveform. We study the self-excitation of the PSR in a geometrically symmetric, electrically asymmetric capacitive argon discharge using PIC/MCC simulations as well as an analytical model. The results show that increasing the number of subsequent harmonics in the driving voltage waveform enhances the asymmetry and, therefore, leads to a significant increase of the current amplitude of higher harmonics, which are generated due to the nonlinearities of the sheaths. These high-frequency resonance oscillations between the capacitive sheaths and the inductive plasma bulk can only be reproduced correctly by the analytical model, if the cubic sheaths chargevoltage relation and the temporal modulation of the bulk length and electron density within the RF period are taken into account. Furthermore, we demonstrate that the nonlinear electron resonance heating (NERH) associated with the presence of PSR oscillations significantly contributes to the total electron heating and causes a spatial asymmetry of the ionization.
Bulletin of the American Physical Society, Oct 11, 2016
of Sciences-Self-organized spatial structures in the light emission from the ion-ion capacitive r... more of Sciences-Self-organized spatial structures in the light emission from the ion-ion capacitive radio frequency plasma of an electronegative gas (CF 4) are observed experimentally by Phase Resolved Optical Emission Spectroscopy for the first time. Their formation is analyzed and understood based on particle-based kinetic simulations. These "striations" are found to be generated by a resonance between the external driving radio-frequency and the eigenfrequency of the ion-ion plasma that leads to a modulation of the electric field, the ion densities, as well as the energy gain and loss processes of electrons in the plasma. The growth of the instability is followed by the numerical simulations [1]. The presentation introduces this effect conceptually and explains its physical origin. [1] Y.-X. Liu et al. 2016 Phys. Rev. Lett. accepted for publication
We investigate the Ar +-Fe asymmetric charge transfer (ACT) reaction using a combination of plasm... more We investigate the Ar +-Fe asymmetric charge transfer (ACT) reaction using a combination of plasma diagnostics methods and a kinetic model of the afterglow plasma, which allow monitoring of the temporal evolution of the densities of different species. The iron vapor is created inside a discharge cell by cathode sputtering; its density is measured by atomic absorption spectroscopy. The rate coefficient of the reaction is evaluated from the emission intensity decay of Fe + * lines pumped by the ACT process in the HeAr -Fe and Ar-Fe afterglow plasmas. The measurements yield a rate coefficient k = 7.6(±3.
For measurements of recombination rate coefficients of slowly recombining ions with rate coeffici... more For measurements of recombination rate coefficients of slowly recombining ions with rate coefficients below 10 −7 cm 3 s −1 in Flowing Afterglow Langmuir Probe (FALP) experiment the plasma decay time has to be long and this requires buffer gas (helium) pressures in range 500-2000 Pa. Application of Langmuir probe for measurements of electron energy distribution function, EEDF (f (ε)), at these pressures is not trivial. The Druyvestein formula for calculating f (ε) from the second derivative of the electron current to the probe (I e) cannot be used. More complex analyses of measured probe characteristic are required to obtain reliable EEDF. To characterize the extent of the required corrections, the probe characteristics in well-defined flowing afterglow plasma were measured and two different methods were used to obtain EEDF. In the first method, the first and second derivative of probe characteristics were used to obtain EEDF [1]. In the second method, numerical correction was calculated [2] and applied to values obtained from Druyvesteyn formula. By comparison of obtained EEDF with Maxwell distribution we determine a region of electron energies where advanced analysis is necessary. It was found that in a helium afterglow at 250 K, with pressure 600-2000 Pa, where plasma is nearly Maxwellian with Te ∼ 250-600 K, the correction is necessary for electron energies ε ≤ 3.5 kBTe. At higher electron energies correction is negligible and is not necessary.
Metastable argon atom kinetics in a low-pressure capacitively coupled radio frequency discharge
Plasma Sources Science and Technology, May 18, 2023
The kinetics of excited atoms in a low-pressure argon capacitively coupled plasma source are inve... more The kinetics of excited atoms in a low-pressure argon capacitively coupled plasma source are investigated by an extended particle-in-cell/Monte Carlo Collisions simulation code coupled with a diffusion-reaction-radiation code which considers a large number of excited states of Ar atoms. The spatial density distribution of Ar atoms in the 1s5 state within the electrode gap and the gas temperature are also determined experimentally using tunable diode laser absorption spectroscopy. Processes involving the excited states, especially the four lower-lying 1s states are found to have significant effects on the ionization balance of the discharge. The level of agreement achieved between the computational and experimental results indicates that the discharge model is reasonably accurate and the computations based on this model allow the identification of the populating and de-populating processes of the excited states.
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